Ambient air quality and human health : Current concepts Part 1

There is a fundamental reevaluation of the association between air quality and human health taking place. This reevaluation is motivated by several recent developments: increasing interest in air quality as an environmental issue; interest in the unanswered questions regarding the epidemiology of asthma; and the reduced prevalence of the principal hazard to respiratory health, cigarette smoking, 
the control of which invites interest in second-order determinants of health. This article attempts to provide a framework for understanding air quality issues that pertain to human health. The objective is to provide the specialist in respiratory medicine with an overview that will assist in educating patients and in responding to their inquiries, and to equip the physician to respond to requests for assistance or interpretation when called upon to comment on public policy issues involving air pollution. The implications of setting air quality standards or objectives to meet arbitrary levels of risk of health effects are examined. The current state of the art does not support risk-based air quality standards. A policy of continuous improvement is most protective of both human health and the environment.

There is a fundamental reevaluation oi" the association bL'tween air quality and human health taking place.This reevaluation is motivated by several recent developments: increasing interest in air quality as an environmental issue; interest in the unanswered questions regarding the epidemiology of asthma; and the reduced prevalence of the principal hazard to respiratory health, cigarette smoking, the control of which invites interest in second-order determinants of health.This anicle attempts to provide a fra mework for understanding air quality issues that perta in to human health.T he objective is lo provide the specialist in respiratory medicine with an overview that will assist in educating patients and in responding to their inqui ries.and to equip the physician to respond to requests for assistance or interpretation when called upon to comment nn public policy issues involving air pollution.The implications of setting air quality standards or objectives to meet arbitrary levels of risk of health effects are examined .The current state of the art docs not support risk -based air quality standards.A policy of continuous imprnvement is most protective of both human health and the environment.Key Words: ,\ir /Jn//111io11.i\ir ,111alil_ l'.I /rn/1/1 Q ualite de l'air ambiant et sante des humains : Notions courantes -Premiere partie RESUME : On proccde actuellemcnt ~ um: reL'valuation l"undamentale de l'association entre la qualite Je I 'air et la sante des humains.Cette reevaluation est motivce par plusieurs faits rccents tels qu 'un interet accru pour la qua lite de I 'air comme question environnementale, un intcret pour les probli:mes non rcsolus en matiere d'cpidemiologie de J'asthme et, la diminution de la prevalence du principal ri •que pour la sante respiratoire, le tabagisme.dont le controle convie i\ s'interes •er aux determinants de la sante de seconde cate.gorie.Cet article essaie de foumir un cadre propicc i1 la comprehension des questions en matiere de qualite de I 'air ayant un rapport avec la santc des hunwins.L.'nhjectil" est dL• fonrnir au spfrialiste en maladies respiratoires une vue d'enscmhk qui Jui scrvira pour eduquer ses patients ct repondre a lcur,; questions.ct de preparer le medecin i1 faire face aux clcmandcs LL1ide ou d'ex pl ications sur la politiquc publique en maticrc de pollution atmospherique.Les consequences de l'etablisscment de normes de qualite de l 'air ou d' ohjcctifs pou r satisfaire des niveaux de risquc arbitraires son! examinees.Actucllemenl.!es rcsultats des recherches Jes plus elaborces ne son I pas en favcur cle normcs sur la qualite de !'air basccs sur le risquc.Linc politiquc d"am.:li1>rationcontinue s'avi:.reune meilleure protcc1ion a la fois pour la santc des humaim ct pour l'cnvirnnncmcnt.
A IR QUALITY IS UNDERGOlNG A RE-EVALUATION AS A RISK factor fo r human di sease.On a scientific level.this renewed interest is evident in the intense interest and discussion !hatcentres on the effects of sma ll particulates (the so-called ' PM 10 fraction•.or particul ates be low the cutoff of the respirahle range of IO ~tm). the relationship between ozone and asthma.and recent studies documenting health effects in extreme situations in developing countries and countries of the former Soviet Union and Warsaw Pact ( 1.2).In Canada.thi s increased ac tivity is man ifested hy voluntary multistakeholder programs such as the industry-sponsored Accelerated Reduct ion/El imination of Toxics C ARET) program, which deals with air and other media, government in itiatives such as the NOx!YOC Management Plan and the federal Green Plan for a Healthy Environment, and a series of recent initiatives linking air quality and energy policy, includ ing the 1993 Comprehensive Air Quality Framework for Canada.T here also seems to be increas ing activ ity on air quality issues on the provincial and local levels, especially the ambitious multistakeholder Clean Air Strategic AlliancL~ in Alberta.
Thi s level of activity on air quality issues was not evident just fi ve years ago .A common thread seems to be that air quality is not so visibly separated from other env ironmental health concerns as in years past.As wel l, there seems to be an increasing emphasis on documenting the cost of the effects of exposure to air pollu1io11 compared with the cost of air quality management, a trend particul arly evident in the case of PM 10.Another trend, not documented in the li terature but ev ident from observation of numerous heari ngs and public meetings, is that the issue of air pollution appears to be fused in the public's mind with that of the perceived increase in frequency of asthma and mortali ty from the disease.
T here appears to be a difference in perception on e ither side of the border with respect to ambient ai r qua lity as an environmental issue.In a 1992 su rvey conducted fo r Health Canada, Slovic et al (3) ranked various health ri sks as perceived by Canadians.•outdoo r' air quality , as a survey item , showed a paradoxical response.Although it was one of the lower ranked health risks for which respondents attributed a "high' health ri sk (20%) , it was one of the more highl y ranked risks for which respondents attributed a ' moderate ' risk (approx imatel y 50%).' Indoor' air quality showed a similar pattern, although it was less highly ran ked as a high health risk.However, other items that incorporated a ir quality issues, particularly •ozone depletion ' , 'chemical poll ution' and 'cl imate change' , were ranked much higher (4).T his suggests, but does not in itself prove, that the Canad ian publ ic now sees air quality less as a particular, well-defined issue and more as an aspect of a much greater issue involving chemica l pollution, envi ronmental health and ecosystem stabi lity.By contrast, American jurisdictions as di verse as California and Vermont have ranked ambient air qual ity among the envi ro nmental issues of greatest health ri sk to res idents in risk comparison projects, and have done so separately and in addition to more aggregated categories of environmental risk (5.6).This suggests that Americans continue to see am bient air quality as a relati ve ly discreet issue.The difference may re flect differences in the perceived severity of urban air pollution in the major centres of each country.
The renewed emphasi s on air quali ty issues suggests that it may be timely to review current concepts on air pollution and human hea lth effects, particularly for the bene fit of physicians who may find themselves in a position of responding to questions or educating the public.A review of the topic may also be use ful in shaping clinical judgements about what is and what is not like ly to be a response to air pollution in a particul ar patient, something that is very di ffic ult to say with certainty.

BACKGROUND
In the 1950s and 1960s, the identification of outdoor.or ' ambient' , air qual ity as a possible ri sk factor for respiratory disease initiated a new field of investigation in air quality studies (7-1 2).This led to a series of st udies on human health effects of exposure to air pollutants, the most extensive of which were know n collectively as the Community Health and Environmenta l Surveillance System (CHESS) studies, to be disc ussed in greater detail in part 2. T he studies L 'Onducted during that era remain the basis for most American ambient air quality standards and many Canadian air quality gu idelines.Exceptions are the standards for ozone, which were based on human experimental studies after an initial standard for total oxidants, based on epidemiological data, and PM Io, which was revised from an earlier standard for total suspended particulates dating from this era, in part because of epidemiological ev idence that the PM 10 fraction was more important than total particulates incl uding those of larger size (1 3).
T he first United States legislation dealing with air pollution was passed in 1955 (14).T he C lean Air Act was passed in 1963, amended in 1970 and has been modified several times since.The 1970 amendments, which gave responsibility for administering the Act to the then-new Environmental Protection Agency, began a process of general continual improvement that lasted fo r over a decade.Until about 1982.there was substantial progress in controlling air pollution, particu larly total suspended particulates (the first to come under reasonable control nationally in the United States) and ox ides of sulphur.Less progress was made with photochemical oxidants dur ing this period.A National Commiss ion on Air Quality convened in 1981 concluded that.although the overall gains in air quality we re more modest than had been expected, the significant effect of the C lean Ai r Act had been to keep the si tuation from getting much wor~e ( 15).However, the worst problems seemed to be localized to only a few large urban centres, in pa11icul ar Los Angeles, New York, Philadelphia, Chicago, Denver and the Ohio River Valley.Within a few years, however, air pollution levels, particu larly oxides of nitrogen and carbon monoxide, were again increasing in major metropolitan areas of North America as a consequence of increased numbers of vehicles on the road, des pite improved control of emi ss ions from individual vehicles ( 16).During the 1980s, little further progress seems to have been made in the Un ited States towards tighter regulation be1.. :ausc of competing political priorities.One exception has been Los Angeles, which has made substantial progress in rt'ducing oxidant levels from previous levels ( 17).Howcwr.that city continues to haw, despi te its progress.the most refractory air quali ty problem in the United States.
In the early years the Canadian problem seemed to be almost trivial compared with that in the United States.However.there were early voices of concern.Bates ( 18) was among the first to consider the implications of air quality studies for Canada.particularly in a widely circulated book l'ntitled A Citizen's Guide In Air Pollutin11.Lac ki ng data on pollutant levels and personal exposure in Canad ian centres, Bates drew inferences from the United Kingdom and the United States in an effort to describe what might have been happening in Canada.Bates also drew attention to Swedi sh models as perhaps having relevance to the problem in Canada.He observed at the ti111e that ''For every physic ian concerned about ... air pollution.there are maybe 50 biologists whose interest relates to the conserv ation of wildl ife and the possible long tem1 effects of detergents and pesticides on the countryside and the animals it supports.T his has had the effec t of diverting attention away from human problems.and deserves some correction." Parliament passed the Clean Air Act in 1971 , putting this country on track fo r red uction of emiss ions.Canadi an provinces, however, have primary jurisdiction on environmental issues, and they enacted ambient air quality standards of thei r own, more or less in concert. in the ensuing years.The federa l government retained authority in emissions of speci fic tox ic substances (so-called 'air toxics') and fuel composition ( 19).
Air pollution levels did generally decline in Canadian cities in the early 1970s (when most were at or near the curre nt maximum acceptable guidelines).espec ially leve ls of sulphur dioxide and suspended particulates.but tota l oxidants ~howed little change or even increased in southern Ontario; nitrogen dioxide began and stayed relatively low (20.2 1 ).This trend tapered off in the later 1970s in most cities except Montreal.Not surprisingly, given the source of emi ss ions.particulates and sulphur dioxide predominated in the East and oxidants generally in the West, although oxidants were a problem in Toronto and were noted to be increasing in Hamilton (20).At the time.there was also evidence that acute respi ra tory illness was assoc iated with air qual ity in Hamilton (22).Other cities may have shown similar effects had they been studied (22).
Subsequently, indoor air 4uality became a major public health issue in the 1980s (23 ).seemingly eclipsing ambient air quality for a while.The declining rate of cigarette smoking seems to have thrown some attention on non smokingrelated risk factors.As the primary detenninant of respiratory health comes under increasing control.invest igators are invited to give their attention to other contro llable heal th risks as a strategy for making further improvements.
Sevt'ral recent devclopmrnts have contributed to the renewed recognition of air quality as a public health issue.One is a series of landmark studies conducted by Bates and col -Can Respir J Vol 2 No 4 Winter 1995 Ambient air quality and human health leagues in the late 1980s dunonstrating an association between increased frequency of hospital admissions, many fur asthmatic episodes, and sulphur dioxide or airborne sulphate in southern O nt:irio (24) and Ya11L'ouvcr (25).This observation coincided with the demonstration of increasi ng mortality from asthma in many countries (not necessa rily Ca nada) (26) and suggested a plausible association with increasing lewis of air poll ution, T here soon followed a series of studies from Holland linking episodes of asthma with exposure to ozone (27 .28 ).Most recently, the outstanding example of air quality-related health research in North America -the Six Cities Study -reported that exposure to airlx1me particulates of respirable size was associated with rno11ality (29).All of this activity has contributed a sense of urgency to air quality issues.
The Canadian Smog Advisory Program is a cooperative federal and provincial prog ram designed to monitor and advise on impl ications and policy in nonattainment areas .It may advise a health alert (as occu rred once in Toronto in 1994) and may devise indicators to monitor progress.It is most active in the Windsor-Quebec corridor but is expected to extend its activities soon to Brit ish Columbia.
The current situat ion in Canada is highly variable among regions.In general.ambient ozone is the most con1111011 exceedence and e pisodes in excess of the one-hour maxi111um acceptable objective are most common in the Windsor-Quebec corridor, the Saint John area and the lower Fraser Valley.T he highest frequency of exceedcnn:s involving ozone in Canada coincide with concentrations of vehicular traffic and of the petrochemical industry, ie. in Windsor, Sarnia, Tonmt<i and Montreal (30,3 1 ).Because ozone is a secondary air pollutant produced from photochemical reactions involving hydrocarbons and oxides of nitrogen, the key to controlling ozone is to control vol atile hydrocarbon emissions and emissions of oxides of ni trogen.
Many recent developments have affected the management o f ai r quality and policy initiatives on the part of federal and prov incial governments.On a political level, the federa l government targeted air quality in the G reen Plan for a Healthy Environment ( 1991) and deve loped its 'NOx/VOCs ' strategy in 1990 to control emissions of oxides of nitrogen (NOx) and volatile organic compounds (VOCs) and , by doing so, to control atmospheric levels of ozone (30)(31)(32).
Among the provinces, Albe rta has lk-rnonstratcd particular concern over the issue because or its rok as a leading energy exporting province with a heavy econom ic stake in matters directly related to air 4uality, such as fossil fuels and publ ic transportation policy.In Alberta.interest in air quality incl udes ambient air 4uality, sour gas and hydrogen sulphide em iss ions , acid deposition and hazardous air poll utants ( 'air tox ics', which are understood to mean any potentially hazardous chemical that may be emitted to air and that is not covered by an existing ai r quality guideline).A provincial initiative in the late 1970s and early 1980s called the Acid Deposition Research Program (ADRP) sponsored a great deal of useful scientific work on air quality.There was no immediate successor to ADRP, but in 1990 a new mechanism was set up to deal with unresolved issues.The Clean Air Strategic Alliance of Alberta (formerly the C lean Air Strategy of Alber1a) is a government-industry partnership desig ned to address air quality issues in the province.As well, the Energy Resources Conservation Board has been particularly active in addressing issues of accidental release of hydrogen sulphide from sour gas deve lopment.There has always been concern in the province over point-source emissions of hazardous air pollutants, particularly hydrogen sulphide from sour gas development.

AIR QUALITY
Air quality is more than a statement regarding the ex tent of air pollution at a given time.Air quality is also a judgeme nt regarding both the effects that we perceive and effects that act below the threshold of human perception but that may affect the ecosystem and so indirectly affect human life.
Public policy discussions regarding air quality tend to equate the term with simple concentrations of air pollution and are usually dominated by concerns over human health.l lowcver, for many air pollutants, particularly the oxidants and airborne acid-forming chemicals, the direct effects of air pollution on vegetation and freshwater biota are proportionately much greater than those demonstrated on human health.Many effects of air pollution te nd to act over prolonged periods or arc consequences of repeated exposure.Air qua lity must therefore be described in te1ms of relevant time intervals.not just as instantaneous descriptions of levels of air pollutants.Air quality can also be perceived as a description or an end-point achieved by the action of numerous variables, some or them difficult to quantify.that are responsible for detcnnining the final chemical state of the air we breathe.
One way to approach the interpretation of air quality is to compare it with an ideal state of "clean air'.

What is 'clean air' '?
A particularly useful de finition of clean air has been developed by a working group of the Clean Air Strategy for Alberta (now the Clean Air Strategic Alliance) in order to achieve consensus and a reasonable working vocabulary among representatives of government, industry and the public CD):

r" is rl'prcscnlcd hy air Iha! is ('Ssc111iully odourless, tasteless . looks dear and has 110 measurable short-or longtam adrcrsc ctft•cts 011 people , animals and the enl'iro11men1.
This definition makes several distinctions that are important from the standpoint of public policy and air quality manage ment.One is that clean air is a socially ag reed upon objective with definable charaetcrist ics.not an absolute or scientifically definable state of chemical purity.Clean air approximates the natural state of the atmosphere free of' human intcrkrence or exceptional natural emissions (as by fire or volcanic eruption).This definition is a response to a point of view common in regulatory and public policy debates.that clean air is basically an atmosphere that does not exceed air pollution standards.
This point of view is increasingly out of touch with the public's environmental awareness, which expects progress towards cleaner air and reflects concern over current levels of air pollution, regardless of what may be permitted by air quality guidelines.
It follows from this definition of clean air that 'dirty air'.or air pollution , is any significant deviation from this state of clean air as a result of human activity.This concept comes close to the view he ld by the general public.which seems to be growing increasingly impatient with reassurances that health problems cannot be linked to obvious pollution levels.Some authorities speak of air pollution as any degradation in quality causing an adverse effect or loss of utility to humankind.whether the cause is human activity or natural.noncatastrophic processes.such as plant-derived emissions or (distant) volcanic e ruption.However, this definition is problematic in air quality studies because natural background variation is uncontrollable, may exceed levels attributable to human activity and is the natural state in which the local ecosystem evolved in the first place.
Air pollution can be defined as follows: the emission into the air or the production of chemically active compounds at a rate that exceeds the capacity of natural processes to convert or dissi pate them.This is a specific application of a more general definition of pollution (34).Air pollution affects health when these compounds accumulate to concentrations capable of producing a biologically significant effect.Unfortunately, the 'loading capacity' of the environment is exceedingly difficult to measure.and the critical rates of emission that result in accumulation can only be estimated.Also, not all significant pollutants are emitted to air -some of the most important (such as ozone and nitrogen diox ide) are secondary pollutants generated by chemical reactions in the atmosphere.

Measuring air pollutants:
As mentioned above, air quality is more than a simple description of the concentration of air pollutants at a given time.However, the measurement of air pollution is a critical part of the description of air quality.O nly by identifying the concentration.trends in time and distribution of air pollutants is it possible to desi gn effective strategies for the control of emissions from sourcl'S of air pollution.
Only a small number of chemicals known to he present in the atmosphere as air pollutants are actually measured by most agencies respons ible for environmental regulation .The cost of measuring trace levels of minor air pollutants is very large and the additional monitoring capacity is not u •ually worth the additional expense.Instead, air pollution monitoring tends to rely on these basic principles: first.air pollutants tend to go up and down together in a predictable way -it is usually not necessary to measure bcnz(a)pyre ne directly, for example, if one knows the ambie nt hydrocarbon levels; second, accurate measurement of individual chemicals at a particular time is usually much less important than trends over time -the monitoring: technology has therefore been developed for continuous monitoring, not for automated detailed analysis: and third.human health effects are generally attributable to a small number or air pollutants that arc among the "primary pollutants• measured routinely.This assumption \\as inrnrrect in the case of lead, which was not o riginall y a primary poll utant.
Air pollution monitoring takes two general fo1ms: monitoring of ambient air quality and surveillance of particular emissions sources.T he tec hnology for each is generall y different.Monitoring ambient air qual ity is funda mentally a problem of detecting dilute concentrations of particulates or gases in a complex atmosphe re; the p rincipal concerns arc precision, not accuracy.and documenting trends over time.Surveillance of particular e missions sources, on the o ther hand.involves detection of much g reater concentrations but requires greater quantification to a llow the calculation of the mass of emissions fro m a particular stack or veh icle; the princ ipal concerns are accuracy and extrapolation o f dai ly or annua l emissions.Ambie nt air quali ty mon itoring will be disc ussed fi rst ( 16,35).
The technology of ambient air 4uality moni toring has become quite sophisticated in recent years but it still relies o n a .et of instru me nts in tande m sampli ng the atmosphere and recording the resu lts electronically, allowing quick calculation of mean concentrations over relevant periods (typica ll y an hour.a day, or a year).These means are then compared with existing air q uality standards or (i n the case of Canada) guide lines.If the observed mean is g reater, there has been an •exceedence' .In the past, observed le ve ls below the standard or guideline have been di smissed as inconseque ntia l, but in recent years inc reasing public inte rest in air quality and increasing scientific c riticism of the basis of standards has drawn more attention to the absolute levels of air poll ution, not just level s relati ve to an arbitrary standard.
Particulates are typ ica lly sampled today by passing air thrnugh a fil ter and repo rted by count or weight of particl es, as total suspended pa rtic ulates.Increasi ngly, air quali ty monitoring agencies are also re po rting the concentration of particulates below the cutoff of the respirable range of IO µm (the so-called 'PM 10' fract ion), because of the relative importance of this frac tion in human hea lth.Historically, there were ma ny other meth ods for measuring ambient particu late levels.Most are no w obso lete, but the most important was a melhod used in the United Ki ngdom in the early years of air poll ution stu dies, called the British Standard Method, or •Britis h smoke' .Th is invol ved a sampler using m uch s lower tlow rates than American 'high-vol ' technol ogy and was pres umed at the time to be a more accurate reflection of particulates in the respirable range because the larger particles were removed gravimetrically.
The technologies used fo r moni toring gaseous air pollutants are dete rmi ned by the chemistry of the air pollutant.Carbon monoxide is easy to measure because the gas absorbs light in the infrared range; it can be measured continuously with an infrared spectrophotometer designed for the purpose .Highly chem ica lly reacti ve air pol lu ta nts, such as ozone a nd ox ides of ni trogen, are also rel ative ly easy to measure conti nuously; both of these chemicals are monito red by chemiluminescence, a method that detects lig ht emitted when the atmosphere is introd uced into a chamber with a reactant gas .Sulphur oxides, on the other hand , are more difficult to Can Respir J Vol 2 No 4 Winter 1995 Ambient air quality and human health monitor by continuous me thods.There a re two common methods of measurement for sulphur oxides.The 'tetrachloromerc urate absorbe r/pararosan ili ne' method measures to tal sul phur compo nents (S0x).The more recent method of chromatograph ic separation with fla me photometric detect ion.which is also known as 'pulse florescence', separates individual sulphur compounds , including hydrogen su lphide, and is preferred for continuous moni toring .Advances in air monitoring of su lphur compo unds have made it pract ica l to investi gate health effects assoc iated with the predominant sulphur species in the atmosphere rather than ass uming that all the effect is due to sul phur dioxide.
Source monitoring, or emissio ns surve illance, on the other hand, is performed primarily fo r regulatory purposes in support of protecting am bient air q ua I ity .Because the concentrations of air po llutants are typica lly much greate r a t the source, less sensitive but more accurate methods can be used.Sampli ng ty pically involves introduci ng a probe into a stack or an exhaust pipe and obtaining a sample over a relati vely short period of time; this is called a ' grab sample' .T he sample may then be a nalyzed by a variety of we t chem ical methods (36) or, increasingly, by highl y accurate methods such as mass spectroscopy o r flame ion spectrophotometry.
Remote mon itoring of air pollutio n sou rces is becoming increasi ng ly usefu l due to advances in technology.The primary application of these methods so fa r has been in monitori ng acid depositio n and air mass movements over long distances by sate llite.However, it may soon become prac tical for local air quali ty agencies to cond uct survei llance of sources quite accurately from a d istance and to do so routine ly.One example of a 'low tech ' application of remote survei ll ance is the use of a ' R inge lmann chart•.which is a set of printed grids aga inst wh ich the density of thick smoke plumes can be compared visually and ranked hy their opacity as a surrogate measure of particulate density .
Summary indexes of air q uality have been deve loped in order to reduce a complicated profi le of air poll utant concentrations , varia ble over time, to a sing le derived number or pane l of numbers .The intention has been to simpli fy presentation of air quality trends in order to fac il itate monitoring, public education and making policy decisions.One problem with summary indexes of air quality is that they usuall y are di fficu lt to re la te to adverse hea lth consequences because they are normally based on air quali ty standards, not tox ici ty levels.An examp le is Env ironment Canada's 'Index of the Qual ity of Ai r' (IQUA), wh ich is a summary scale from Oto 125 used to communicate air q uality informati on to the public.The IQU A is based on National Ambient Air Q ual ity Objectives and trends are based on the summation of all five (37).It is not used in setti ng publ ic policy or moni toring air q uality trends in respo nse to control measures.Despite considerable attention to the problem and ingenuity in constructing indexes, ai r quality indexes have not fig ured prominently in most management schemes.
An alternati ve approach, at least for public a wareness.has been the use o f 'smog ale rts' (38).T hese have been most usefu l in areas of intense air pollution , such as southern  Ca lifornia.where periodic alerts seem to have a rein forci ng effect on publ ic awareness.Alert leve ls are typically based on some notion of adverse health conseq uences and are accompanied by health adv isory notices, usuall y advising people to stay indoors or to avoid exerc isi ng out of doors .In most jurisdictions where alerts are in use, such as Los Angeles.they arc driven almost ent irely by oxidant levels.

Determinants of air quality:
As noted above.air quality is more than the final concentration of air pollutants at a given poi nt in time.However, to appreciate the com plexities of air quality it is necessary fi rst to understand the factors governing the dispers ion of airborne L'missions.
T he accum ulation of chemically acti ve com pounds in the atmosphe re is greatl y affec ted hy land feat ures and by atmospheric movements.Va lleys, closed-in mountain ranges , and lack of open space (parks , forests, wilderness areas, bodi es of water) strongly increase the severity of air pollution in a given locale.These features hold the ai r mass like a container and prevent dilution and mixing.Stagnant air masses may receive emi ssions for days on end.When wi nds are cairn , a thermal layer of warmer air may sit on an air mass like a lid on a jar and prevent its movement.Th is is called a •subsidence inversion ' .Inversions are situations where the temperature of air increases with altitude rather than decreases, as is usually the case in the troposphere.Subsidence inversions are common almost everywhere but tend to be frequent and long lasting only in a few areas.princ ipall y in narrow river valleys or the seacoast and over large bod ies of water.The combination of hi gh population density.concentrated em issions sources, enclosure by moun ta in • and freque nt inversions explain why air poll ution has been a particular problem in valleys and basins (39 ), such as the greater Va ncouver area and the Fraser Valley and on the shore or inland lakes, as in the case of Hamilton and Toronto (20).These same factors explain why southern Californ ia has a particularly severe problem or air quality .
The dispersi on of air pollutants fro m a si ng le source and the behaviou r of air po llutants in a g iven ai rsheJ can both be mathematicall y mode lled by computer mode ls of greater or lesser comp lex ity (40).T he most familiar such mode l used in western Canada is called •Gascon 2 ' .It is useJ princ ipally to model the probable behaviour or a plu me of gas escaping from a sour gas well or pipeli ne leak.
Figure I describes the situat ion in a typical.simple emission from a stat ionary source to the atmosphere.J\ jct of (usually) wa rm air escapes from the stack at a cert ai n vclocily and is displaced by wi nd , and the plu me describes a Jistor1ed cone wi th one side constrained by the inversion layer and thc other by imp ingement at ground level, where air pollution is detectable on lhe ground.W it hin the cone (pl ume) there is mixing and d ilution over di stance.The higher the stack or the stronger the wind , the longer the di stance until thc pl ume touc hes ground and the more dilute it will be.In the absence of wind , the plume will rise and spread under the invers ion layer until it dissipates.An exce ptiona lly strong wind can cause tu rbulence and a "downwash ' that brings the pl ume down to ground level for short periods.Sou rces close to the ground, such as brush fires or motor veh icles.will contribute to pollution at ground leve l, espec ia lly if the invers ion layer is low.Under other conJi tions.air pollution mode lling becomes extreme ly compl icated (40) and is beyond the scope of this arti cle.
II is a mistake to th ink of air quality narrowl y in terms or ai r pollution levels and to treat air po llution cont rol as primaril y a problem in eng ineering and chemistry .Air pollution leve ls at a g iven time are rhe res ult of these many fac tors acting on the combined emissions from stat ionary and mobile sources, wit h loca l distortions introduced by topography and wi nd patterns.However, ai r quality in ge ne ral a lso ta kes into consideration the changes in air poll ution levels over time, the d irection of exposure at given leve ls. the effec ts of exposu re on the natura l ecosystem and hu man healt h. the effec ts of e xposure on the local economy and agricult ure, and the forces set into play for the abatement of air pollution.The latter brings into the equat ion a list of behavioural facto rs and affects collective leve l o f concern and understanding in the soc iety producing and affected by air po lluti on .Air pollution is tlw product of socia l acti vit ies, the result large ly o r economic activity and transportati on pl an ning.Air polluti on control and the maintenance of air qu al ity is likew ise a soc ial process, involvi ng education.the perception of accidents.economic costs and po litical will.
Figures 2 through 5 bui ld a conceptual model uf air quali ty in keeping with the broader view of the problem.Figure 2 presents the first step in a general conceptual model of air q ua lity.T h airshed is defi ned hy the topographic fea tures of the region aml the atmospherL'.above it.The space that defines the potential volume or air mi xi ng is confined horizontall y by topographic featu res and verti ca lly by the height of the inversion layer.Mobile and stationary sources of em ission release pollutants into the airshcd .T he acc umul ated primary air poll utan ts are acted upo n by sunl ight, driv ing photochemical reactions that fonn secondary poll utants.T he second ste p (Figure 3) is m ix ing and d ilution of both primary and secondary air pollutants within the vol ume of the air mass, greatl y acce lerated by hi g her w ind velocit ies.The third step (Figure 4) in the conceptual model of air qua li ty is doc umentation of the effects of exposure.Healt h ri sks, eco-~ystem damage and materials degradation as a resu lt llf ex posure to air pollutants are observed.and reg ional conce ntrations of' ai r pollutants are monitored.Ti me and variat ion in concent ration became critical variables in determi ning the effects of e xposure.on the nat ura l ecosystem, the economy , agriculture and human health .Thus fa r, the conceptual model has bee n physi cal and chemical.To understand fully the i111plications of air pollution and to return to the definition of clean air advanced above, a social dimension needs to be factored into till' fourth step (Figure 5).WhL' .n costs become apparl'llt and esthl'lic values are impaired, there is a soc ia l response, fac ilitated by research and education.The socia l response is mani fested through pub! ic policy, econom ic incentives, e ngineering controls and education.Air quality improvement is drive n by cost, pe rception, education and public concern translated into political w ill and economic incentives in the marketplace.luclividual beha viour changes, as in the choice of whethe r to dri ve o r take the bus or walk to a near destination.Howe ve r, individual change alone cannot he relied upon to achi eve clean air beca use the individual lives in a complex socie ty in which alte rna tives for tra nsportation and power arc not always readily ava ilable, and there is little that the individual can do abo ut the production and di stribut ion of energy and products that support modern urban l i k .Rather, major changes may be required in transportati on po lic y. technology of tra nsit and fuel s ( 41) and la nd-use planning.

TYPES OF AI R POLLUTION
Tabl e l compares six general types of community air polluti on, as defined by d ifferent chemical characteristics .distribution and sources, a lthough featu res of eac h type may be present to varyi ng degrees in any given com munity ' s air pollution.
Reduc ing. or 'acid ' .air pollution is caused by the emission of sulphur diox ide and particulates.which ac t as chemica l reducing agents in the atmosphere (42).This is by far the oldest type of air pnllut ion, characterized as • t'umifug ium' by John Eve lyn in 17th-century Londo n.This type of air pollution was responsible for most of the early air pollution catastrophes and for the infamous London ' particulars• or •pca-soupers' .which were occasi onall y lethal combinations of smoke and fog that occ urred np to the late 1950s ( 43).Emiss ions of sulphur dioxide are caused by burn ing fo ssil fue ls that contain some sulphur; em issions of partic ulates occur most heavily when combustion is inefficient.Reducing air pollution is produced primarily by stationary combustion sources, such as fossil fuel pown pl ants.industrial furnaces and home heating unit •.Ty pical conce ntrations of air poll utants are hig her in reducing air pollution than in other types, and there is a strong link with acid de position when thi s type of air pollution is transported long distances.Reducing a ir pollution was characteristic in the pas t of o lder areas of industrial concen tration .particula rl y in steel manufacturing ct"ntres such as Hamilton .Thi s type of air po llution is rl'latively easy to control with conventional engineeri ng contro ls.lt remains a seri ous problem in devl'l oping countries, parti ' -• ularl y China .
As citi es in dcwlopt"d countries hav e controlled reducing 218 Furnishings Cigarette smoke Sarn e as 1 and 2 Transported over long distances and may cause environmental effects or, possibly, health effects Most human activity, including industry and ag ricul ture air poll ution.photochemical smog has emerged as a secondary problem.'Smog ' -smoke plus fo g -wa s a term ori g inally applied to vis ible air pollution in the United Kingdom.hut since the 1950 • it has come to be ap plied almost exc lusi vely to photochemical air pol lution (44).
Photochemica l air poll utio n, or smog, is much newer in human hi story tha n reduc ing a ir po lluti on (9.45-47).Smog resu lts from compl icated chem ical reactions in the atmo.-phere that are driven by the energy in sunlight.T hese reactions were first characterized by the pioneering atmospheric chemist Arie Jan Haagcn-Smit, working at the California Institute of Technology in the 1950s and 1960s (9, I l ).O xides of nit roge n produced by combustion and hydrocarbons undergo reactions to produce ozone, specific secondary reaction products of nitrogen, and aldehydes, all o f whi ch are hi ghl y reactive and chemically oxidizing.Smog is caused pri marily by emissions from mobile sources.overw he lmingly automobi les, but it is exace rbated by certain types of e miss ions from stat ionary sources.such as hydrocarbons from re fineries.evaporated gasoline and dry c lean ing solvents , and oxides of nitrogen fro m po wer plants (43 ).lkcause of the need for sunl ight to drive these chem ical reactions a.nd because o f local re liance on the automobile fo r transpo rtation.photoche mical air pollution appeared fi rst in the cities of the Ameri ca n sunbcl t. espec ially Los Angeles.but soon appeared in the northeast region as we ll (48).This type of air pollution has become much more common in de ve loped countries such as Canada , rega rdless of local concentrations of industry or climate.
Table 2 compares the characteristics of reduc ing and o xidi zing or photochemical air pollu tion.
A third type of rnmmu nity air pollution is point-source emissions.Industrial activities or acc idents may re lease a re lative ly large quanti ty locally or a particular type of ai r poll ut ion that becomes a loca l problem affectin g the immedi-ate vicinity of the plant, hut does not usua lly involve atmospheric reactions to any g reat extent.Exa mples include lead and arsenic in the vicinity of a smelter, hydroge n sulphide from a sour gas leak, pesticides from agricultural applic ation, and concentrated fumes from a spill or tank rupture.Such emissions may he the result of accidents, particular ly rel ated to transporting hazardous substances by truck or train.
Acid de position (or 'acid rain') is a general term for long range trans port of acidifying chemica l substances, principally sulphates and oxides of nitrogen.This problem has hecn exacerbated by efforts to control ground-level pollution by building higher stacks; the pollutants rise much higher and are transported much longer distances (49).In recent years, with more effective control of sulphur emissions, oxides of nitrogen have become increasingly more impo1tant in acid deposition .Acid deposition is an international problem, as emissions from the Ame rican Midwest travel freely over Canada, and emissions from Germany and Central Europe fall on Scandinavia.The principal disturbances caused by acid deposition appear to be ecological, with acidification of lakes and effects on sensitive species of trees (50).However.some recent studies have suggested human health effects also occur in the form of increased frequency and severity of asthmatic attacks (5 I).
Indoor air quality is a substantial topic in its own rig ht, wi th great implications for inuividual health (52).A thoughtful consideration is beyonu the scope of this review, which emphasizes ambient air 4uality.
Global atmospheric changes include the accumu lation of •greenhouse gases ' , the consequent phenomenon of global wanning and stratospheric ozone depletion and the e ffects of increased incident ultraviolet radiation at the Earth's surface.In these cases, the effects on human health are indirect.The principal greenhouse gases are in themselves nontoxic.The problems associated with global warming pertain more to changing climate, with implications for agricu lture and vector distribution, and chaotic weather conditions.Like wi se, the chlorofluorocarbons that arc primarily responsible for reduction of the protective ozone layer in the lowe r stratosphere have very low intrinsic toxicity, but increased incident ultraviolet radiatio n may lead to increased incidence of skin cancers and possibly immune effects.T hese issues are beyond the scope of this review hut are discussed in detail elsewhere (49).

TOXICOLOGY OF AIR POLLUTANTS
The toxicology of air pollution is exceedingly comple x.There are different ty pes of air pollution, many d ifferent pollutants and g reat variation in individual susceptibility to their effec ts at low, environmentally relevant concentrations.Generally speaking, the toxicological characteristics of most chemicals at high concentrations are irrelevant to their behaviour and effects as air pollutants.Air pollutio n is therefore best approached as a unique toxicological proble m of complex mixtures at low concentrations.
For example, nitrogen dioxide is an oxidant gas, about a 10th as potent as ozone in caus ing hronchiolitis, and less Can Respir J Vol  potent in produci ng a irways e ffects.but with similar efkcts (53,54).At higher concentrations, of the order encountered in occupatio na l e xposures, the effects of nitrogen dioxide include severe endothel ial injury and pulmonary edema; there is even evidence to suggest direct vasoactive effects in the lung (55).Systemic sy mptoms are often reported and a productive cough is not unusual (56).However, at the lower concentrations encountered in amhient air pollution , the effects are much more subtle.including enhanced airways reactivity and compromised host defences in the respiratory tract (54,57).In cell culture, pretreatment with N-acetylcysteine (a precursor to g lutathionc.which inereascs intrace llular antioxidant capac ity) reduces cytotoxicity after exposure to nitroge n dioxide (58).There is also evidence that pre treatment by ingestion of supplemental ascorbic acid prevents the express ion of airways hyperrespon siveness in normal subjects afte r e xposure to nitrogen dioxide, in a time frame too quic k to be due to oxidant injury secondary to inflammation (59) .T hese findings suggest that effects at both leve ls are med iated directly by the oxidant properties of nitrogen dioxide but are concentration-dependent.In short the che mical properties of this, and all other, air pollutants define their to xicity, but the effects obse rved at low conce ntrations characte ristic of ambient air quality are not necessarily the same or weaker versions of the same effects anticipated at high level e xposures (60).T he inhalation toxicology of air pollutants is extremely difficult to study technically.T here are relatively few laboratories in North America equipped to handle inhalation exposures at relative ly low concentrations.In Canada.the principal laboratories for this type of work are at the Bureau of C he mical Hazards in Ottawa, the Gage Institute in Toronto and the A lberta Environment Centre in Yegreville.
Air pollution of the reducing variety has been studied since the 1930s.The major constituents of photochemical air pollution have been extensively studied at enviromnental PM to, airborne levels were generally well within applicable air quality standards (29).There are many additional effeL•ts for each.associated with higher ex posure levels, and many effects have been documented at the tissue or biochemical level (44).
There are two major "burning• issues in air pollution toxicology at the moment: ozone and PM w.Both will be discussed in greater detail.However. it should be noted that there are many other issues involving health effccb and air quality that are under active investigation.Ozone is a •secondary pollutant' , the product of photochemical reactions, some involving nitric ox ide and others involving hydrocarbons.both of which are primary em issions.Ozone is an intense ly reactive and there fore irritating gas.In recent years it has become clear that elevated ozone levels provoke asthmatic episodes, as discussed in greater detail below, although it is not so obvious that oz.one could be a cause of asthma in the first place (61 ).It now appears to he the most critical constituent of the 'acid summer haze ' effect, described in part 2 (24.62,63).Thus, the toxicological significance of ozone in photochemical air pollution has become increasingly more appreciated in n.:Tent years.T he problem is lhat omne is also a natural background constituent 220 in the atmosphere, particularly in mountainous areas, at concentrations approaching one-tenth to one-half of current air quality guidelines and standards, wh ich arc themselves associated wilh demonstrable effects in recent studies.T his means that there is much less of a potential marg in of safety for ozone than for other air poll utants.but it also means that control of ozone is harder in these areas and that enforcement of tight standards may be difficult or impossible where background ozone levels are h igh.Control of ozone i s not simply a matter of conlrolli ng emissions at an identifi able source; it involves controlling all sources of hydrocarbon release and, simultaneously, all sources of combustion emitting oxides of nitrogen.This is a formidable task .There has been good evidence for many yea~ that health effects due to omne can occur at or near existing guidelines and standards (64).l lowever.it is not clear that ozone can be reasonably controlled down to a level of no effect.
The role of PM 10 in causing excess mortality from respiratory and other causes (29) is a mystery.It i s not at all clear what mechanism is responsible for the nonrespiratory death., because the respiratory deaths occur at relat i vely low L•oncentrations.wi thin current air quality guidelines.Neither respiratory nor nonrespiratory mortal ity show a threshold response.So far, the individual risks documented in population studies have been modest but the magnitude of the effect in an entire populat ion is unexpectedly large (65,6(1).Evidence avail able to date suggests that for every IO mg/m 3 increase in PM IO, there is a corresponding increase per day of I % in deaths overall, 1.4% in cardiovascular deaths, 3.4% in respiratory deaths, I % in emergency department admissions and 3% increase in use of bronchodilators hy asthmatic patients ( 6 7).No threshold i s apparent from their data or from other studies (66).Sorting this out will probably he the principal task in air pollution toxicology of the late 1990s.

Can Respir J Vol 2
No 4 Winter 1995 Another aspec t of air pollution toxicology is the characteri,ation of mi xed exposure.It is ve ry common -indeed, it i~ usual -for more than one h azardous ,1ir pollutant to be present at the same time.In som e unus ual cases it is po ssible that two or mo re c he mical e xpos ures will have an interactive or synergistic effect o n the body.leading 10 greater toxkity than would be predicted from adding up their individual effecb.The proble m of mixed exposure is much talked about, hut in smog, an add itive e ffect of oxidants almost al ways ACKNOWLEDGEMENTS: I thank Dr John R Goldsmith for his input and collaboration on a m uch earlier version of this work t 1983) and for providing me with the drawings th at became Figures 2 through 5 in this article.I thank Dr X Chris Le for assisting in hringing my knowledge of current monitoring methods for SO.x up to date.I al~o thank the American Lung Association of San Diego and Imperial Countries, the South Coast Air Q uality Management Di~tricl, the California Air Quality Advisory Commillce.and the Clean Air Strategic Alliance of Alberta, which together have gi ven me a front row seat to observe air quality issues and public policy .isthey have evolved over 25 years.This paper is dedicated to the m •mory of Dr Stan Rokaw.