Ozone : A review of recent experimental , clinical and epidemiological evidence , with notes on causation Part 1

Part 1 of this review is concerned with theoretical issues of ozone dosimetry, animal and cellular studies that illustrate the mechanism of action of ozone on living tissues, and with clinical studies. Animal studies have indicated that there are long term effects from low level long term ozone exposure. Clinical studies involve controlled ozone exposures on human subjects, both normals and asthmatics. Exercise concomitant with the ozone exposure increases the effect of the gas. It is concluded that the induction of an inflammatory response in the airway, both in the nose and in the lung, is the striking and earliest feature of ozone exposure. Current unexplained observations include: the dissociation between the inflammatory and function test response; the mechanisms of ‘adaptation’ and of airway hyperresponsiveness; and the phenomena that underlie the effect of ozone on maximal athletic performance.

T HE 111Hl.lOGRAPHYO ' OZONE IS NOW SO I.E GTH Y.A ' D includes many hunJrcds o r references to work publi shed in the pas t ,ix years, that any sumnwry ha, to be hi gh ly sclcL•tivc.In this review, it is appropriate to emphasize some L'ozone : Une revue des donnees epidemiologiques, cliniques et experimentales recentes accompagnee d 'un commentaire sur la causalite RESUME : La premiere p,1rtie de cette revue portc sur des questions theoriques de dosimetri e de l' ownc, des eludes ccl-Julaires et an imalcs qui illu stren t le mode ri'action de !' ozone sur lcs tissu s vivants et, des etudcs cliniques.Les eludes animal es demo ntrent que de foibles nivcaux d'exposit ion i1 long terme :1 I' ozone cntra111e nt des effets i1 long terme.Ll'S el udes cliniqu cs comprennent des exposi tions eo11tr6lees a J\izonc sur des sujcts humains, sains et as thmatiques.L'exercicc associe a une exposition a !' ozone accroi't l'effct de ce gaz.On concl ut done que !' induct ion de la re,ll'ti on inllarnmatoire dans Jes Vllies aerien nes.dans le nez ou le poumon.est la caracteristique frappant e et la plus prccoce survenant apres une exposition i .J !'ozo ne.Des observati ons courantes mais qui reste nt encore inex pliquees compre nncnt: la dissoL•iation cntrc la rl'ponsc inllammatoire et Jes resultats des eprcuves fonctionncll es; Jes mecanismes «d'ad aptati on,, et d'hypcrreac tivite bronchi411e ct k phcnllmtne sous-jacent a I' effet de I' nrnnc sur la pe1'1'ormance athletiquc max i male.special aspeL •ts or 01011e that ha ve to be undcrstooJ if an informed evaluation of ' causality' is lll be made.The effects of any pollutant have to be L'on~iJered in relation to possible acute e ffects on the one hand.,md lon g term effects on the BATES other.Thus. in the case of sulphur dioxidl.'. the special sensitivity of some asthmatics to this gas at very low concentration might play some part in explaining the epidemiological evidence of inncased hospital attendances in relation to it; however.the question of whether living in a region with generally higher ambient sulphur dioxide levels increases the risk of developing chronic obstructive lung disease is obviously different -and is hardly likely to be explained by the first observation.
In the case of ozone, animal exposures arc valuable in relation to mechanisms of action.although recent human experimentation using bronchial lavage and even bronchial biopsies have made this info rmation rather less relevant than formerly.Animal exposure data are of more contemporary importance in relation to the possible effects of longer term exposures because controlled human clinical studies of that phenomenon arc impractical.Hence, in the following review, particularly close attention is paid to the effects in animals shown to follow exposures lasting over weeks or months.In clinical terms.the complex questions involved in the multifactorial disease of 'asthma• have to be reviewed it• the slrcngths and weaknesses of the prima facie case in relation to ozone arc to be appreciated.This is because, before a judgement of causality is attempted, not on ly must all collateral evidence have been reviewed.but the strengths of what may actually be tenuous linkages in the inft'rcntial argument must also be evaluated.
The question or 'biological plausibility' in coming to a judgement of causality is complex.In the case of ozone, it requires consideration of dosimetry. of differences between animal species.and of the pathology and pathophysiology of common conditions such as asthma.
T hese are the considerations that have determined the comparative emphasis in the following sections.

ANIMAL AND CELLULAR STUDIES Ozone dosimetry
A wnsiderable body or work has been published in relation to ozone dosimetry (the dose delivered to different regions of the airway) during the past few years; only the major conclusions arc surnmarizcd in this section: • Althoug h somewhat different kinetic ass umptions and geometric ;_iirway data h;_ive been used.a ll the models inJicate that in all species the tissue dose pf inhaled ozone (expressed as µg/cm 2 /Jnin/µg of inhaled ozone) reaches a maximunt in the terminal bronchiolar region.The net dose is highest at the first point of impact.namely the trachea.• The maxim;_illy affected 1,onc. in terms of tissue dose.
in the human and in the rat is the 17th generation in the airway.For the human data, this calculation was based on a tid;_il volume of 800 mL anJ a breathing frequency of 15 breaths/min (I).• T he effect of exercise or increased ventilation is to increase the term inal bronchiolar regional dose slightly.and to increase significantly the pulmonary regional total dose and the dose in the proximal alveolar region.
• D ifferences among animal spl.'.Cics have been considered in detail (2): discussion or these is beyond the scope or this review.• Theoretical dosimetric calculations have been compared with measurement~ or ozone in the human respiratory tract under different conditions.Ge rrity ct al ( 3) measured ozone uptakt: in the cxtrathoracic airways of normal subjects and found that di sappearance was as high as 40% on inspiration, and 92 % on both inspiration and expiration.Both of these values fell as respiratory rate was increased.A surprising finding was that there was only a small difference between the uptake that occurred with nose breathing and that with n1outh breathing.Hynes et al (4) found little differL'ficc between nose and mouth breathing in pulmonary function response to ozone.Thest: studies, taken as a whole, indicate that dcpw;ition modelling of inhaled ozone e mphasizes the terminal bronchiolar and centriacinar regions as si tes of maximal ti ssue deposition of the gas.Exercise, by increasing the lung volume and tidal volume and increas ing the breathing frc quency, woul d be expected to increase the tissue dose significantly.T hese factors would be operative in addition tu the higher dose delivered under these cin;umsta11ces.

Acute exposures
T he princi pal effects observed after acute exposures of a variety of species to ozone concentrations less than 1.0 ppm are lung inflammation and c hanges in lung permeability (5-7) and increased airway respons iveness.There is increased mortality if the animal is subsequently challenged with a bacterial aerosol.The initial target of ownc appears to be lung flui d lining components and cell membranes (8) .Reactive oxygen intermediates arc involved in the cell damage.which is partly caused by auto-oxidation chain reactions.Increased permeability has been demonstrated afte r e xposure of rats to 0.12 ppm or 0.4 ppm for two days (lJ) o r in guinea pigs exposed to 1.0 ppm for I h ( I 0).T he inneascd airway responsiveness is generally consiJcrcd to be ; _i consequence of the induccJ inflammation.and O'B yrnl.' and colleagues ( I I) found that nculrophil de pletion inhibited the increased airwny respons iveness after ozone exposure in dogs.However.a recent series of e xperiment.,;on an isolated rat lung preparation ( 12) showed that 01onc could increase airway responsiveness and damage airway e pithelium in the absence of ncutrophils before a microvascuh1r leak was c;_iused.The presence of neutrophils w;_is shown to increase the magnitude of these effects of ozone.
The third effect, to increase mortality in mice if 01.onc exposure occurs before a bacterial aerosol is admini stered.is primarily due to its effect on macrnphages.Recent experiments ( 13) using Str<'plococcus ::.noepide111ic11s in mice showed that 3 h or exposure to either 0.3 or 0.8 ppn1 ozone led to increased mortality.greater in one strain of mice than in ;_inothcr.T he difference in sensitivity was shown to be due to ; _i difference in the sensitivity of the alveolar macrophages in the two different slrains or mice.It is lhe phagocytic efficiency or the macrnphage that is impaired hy the prior 01one ex posure.
Evidence that ozone impairs the clearance mechani sm or the lung was prnvi ded by the observation that ozone ex posures or base levels of0.06 ppm wi th daily excursion s lo 0.25 ppm in rats increased the rete ntion of asbestos fibres ( 14).This was attributed to interfere nce with macrophage func-1 ion.Ozone exposure has also been shown to d ecrease the T lymphocyte and anti viral response ( 15 ).
Ploppcr et al ( 16) rece ntly poin teu out that nonhuman primates a ppear to he more responsive to ozone at conce ntrations less than 1.0 ppm than arc rats.This may be due to differences in nasa l structure (and hence in delivered Jose lo !he lung ) nr to uifferences in lung structure.si nce rats do not possess the many gene rati ons of re spiratory bronchioles that exist in human anu primate lu ngs .
This bri ef su m mary or the effects o f ac ute a nimal e xposures shown in animal experiments ind icates that aggravation of inllammatory lesions , increased airway responsi veness anu worse ning or respiratory infec tions are effec ts that should he looked for in an exposed human population .
Other phenome na arc dcscribcu after ac ute ozone exposu n.:s to animal s or to ce ll s. usua ll y to hi gher concentrations than those in amhient ai r.These incluue: changes in the nasal cavity and naso pharynx ; asscssmC'nls or the effects or the age or the rat nn the outcome -in gc nC'ral youn ger rat s see m more sensitive; no d iffe rences in e ffects between normal rats and rats with claslase-induced emphysema : studies of a wide range or biochemical effects.particu larly on pol yunsaturated fatty acids, antioxidant s and prot e ins. a nd on the production or ,irachidonatc metabol ites; genotoxi ci ty.mutagenic ity and effects on e,ircinogeni ci ty (although such effects can be shown.ror in stance, on DNA J 17 J. the re is no human correlate): and cytogenel ic stuuies showing e ffects with in vivo exposures ll) as little as 0.4 ppm ozone .
Wi tschi ( 18) rece ntly re vi ewed these observations.poi nting out that the radiomimetic activity of lVone did make it a potential contributor to huma n lung cancer.

Long term exposures
Human controlled exposures for longC'r than a few hours arc :mt feasible: hence, the results of longer term an imal expo-sLires arc very importan t.Si nce the primate lung appears to be more sensitive to ozone than the rat lung is. the e vidence fro m primate c,;posurC's (Macacufc1scicularis) will be con sidered first.Almost all of this work originateu in the Primate Center in Da vis, C alifornia.It was rece ntl y elegantly summarized and particularly well illust ra teu by thi s research group ( 19).The exposures were ge nerally to 0.25 ppm otonc fo r 8 h/day , seven days a week fo r 18 months.A ve ry brief summary of their fin uings anu concl u. ions follows: • T he initial inflammatory response is mod ified in subsequent ex posures.but in flammatory ce lls in the peribronehiolar connective ti ssues persist during ex posures that last up to one year.• Epithe li al and interstit ial changes in dista l airways are the most strik ing morphological changes in anima ls exposed for a year or more.There is a proliferation of nonciliated bronchiolar and type 2 alveolar epithe lial cells.These changes start early and arc e vident ,1fter 50 h of exposu re to 0.8 ppm of 01onc.• NC'crosis of ciliated and type I pneumoeytes occurs, and these cells are replaced by the ce lls noteu above.The lo ng te rm effect is therefore a ' re modelling• or the centriacinar airways by extens ion of bronchiolar cell type s in airways that we re formerly alveolar ducts.• There is an increase of collagen locali zed in the peribronchiolar and centriacinar regions.Th is confirms earlier studies fro m the samC' laboratory in rats cxposeu to h igher concentrations of ozone.• The morpholog ic al L"11anges indlll:cu hy the 01lmL' cxposurC' were still prese nt after a six-month postexposurc period of livin g in filtered air.Quantitative morphology studies indicate that the lungs were more abnorma l a fter the post exposurc period than immediate ly a fter the ozone C'xposurc .Rat ex posures or longer than two wee ks haw bcC'n studied in a number of ce ntres.Using a 78-weC'k exrosurc protocol o r a base exposure of 0 .06ppm with spikes to 0.25 ppm.C hang et al (20) showed the following: that an acute response occurred in the centriacinar reg ion.and that lhese changes partly resolved : that type 2 cells inu•eased , and that the interstitium was increased : and that the basement membrane was thicke ned .but that ccntriaci nar remode lling did not occur.Earlier observations from the same laboratory using a six-,veek ex posure protocol of similar concentrations (21) noted tha t there was an increase in interstitium thic kness and an increase in number or al\'eolar macrophages .Ra t exposu res to 0.95 ppm for 8 h/uay for 00 days \Vere founu hy Barr ct a l (22) to have resulted in airway remodelling, ,rnd the les io ns resulti ng fro m chronic versus daily episodic exposures were similar (23).
Costa et al (24) demonstrated changes in lung function in rats after brief e xpos ures.and more reL•cntly repo rted reduction s in force d vital capacity (FYC) of rats artC'r repetitive o zone exposures (25 ).
Saldiva ct al (26) in Sao Paolo expo.,ed 60 rats for six months to the ambient air and compared them with connols kept for the sa me length of time in a clean area./\.monitoring station 200 m from the e xposure lnc,11 ion showed that ozone wen t up to 0.4 ppm.particulates up to ' :JO ftg!n/ sulphur dioxide up to 0.025 ppm ,mu carbon nHmnxidc up to 4 ppm.T he exposed rats hau .,ecretorycell hyrcq1lasia in the airways.ultrastructural ciliary alterations and a more rigid mucus -changes that caused mucociliary clearance impairment.Nasa l resistance and in flamma tory cells in hrnnchoalveolar lavage ( BAL ) ,vere also increased in !he expus<'u group.

Ozone in combination with other chemicals
Since human exposures arc rarely tn a single pollutant.the question of interaction with other chL'mirni.sassumes some im po11ance .In very detailed experiments lll1 rats.Last (27) studied the effects of ozone and su lphuric aciu aerosol alone and in c ombination.On ali lllilL'O!l1esrclatL' d to an inflamma-BATES tory response.the two were additive in thei r effects but were not L•learly synergistic.Ni shikawa ct al (28) stud ied the effec ts or ozo1ic and cigarette smoke in guinea pigs.They showed that airway responsiveness and permeabil ity were affected when hoth irritants were inhaled.although single cxposLtrL'S to each separ,itl'iy at the same concentration produced no effec t.This suggests that there may he a synergistic effect.

CLINICAL STUDIES Effects on the nose
Recent experirnt.•ntshave shown that after ozone exposure.changes in the nose detected by nasal lavage can be used as an indicator or changes in the lung.Graham and Ko ren ('.2l)) exposed IO norma l suhjects to air or 0.4 pp m ozone for 2 h with exercise.Nasal lavage was performed before.immediately al'ter and 18 h arter ex pos ure.BAL was al so dunL• at 18 h.Nasal polymorphonuclear ncutrorhil s (PMN) inc reased 7.7 -fold imml'di ate ly al'ter exposure and were still increased 6-fold at l 8 h.which was identical to the increase in PMNs in BA L at th is time.Albumin levels increased 3.9-fold in nasa l lavage and 2.2 -fol<l in BAL at 18 h.Comparison or the PMN data in nasal lavage and in BAL for each indi vidual showed a significant quantitatiw correlation artcr the air expos ure but not after ozone exposure.
Bascom et al (30) studied 12 asymptomatic subjects with a hi story of allergic rhin it is.They were exposed to 0.5 pp m of ozone in an exrosure chamber without exerc ise or to clea n air as a contro l.Nasal challenge was performed with four doses of ragweed or grass anti gen.Symptoms wen: r:1ted and nasal lavage was performed.Histamine and album in concentrations were measured.and toxoid-a ntitox oid mi xtu re esterase activity and cell counts perfo rmed.Ozone caused a significant increase in upper and lower respiratory symptoms; a mixed inflammatory cdl influx wi th a 7-fold increase in nasal lavage ncutrophils; a 20-fold increase in cosinophil s; a I 0-fold increase in mononuclear cells; and an apparent sloughing of epithelial cl'lls./\lhumi n concentration also increased.However.all or this did not inc rease the response to allergen.
Fri sc her et al (31) recently repurtL•d sllme fi eld stud ies on the nose.Forty-four children were studied from May to Oct,lber 1991.Each had fi ve to eight lavages.One hundred and forty-eight nasal lavages were performed on 14 days follo wing high ozone exposure (more than 180 ~1g/nr' ur 90 ppb) and I 06 after low exposurL'.S ( less than 40 ~1g/m 3 or 70 ppb).A significant increase in PM N counts occurred following hi gh ozone days (27 .38xJO-' vers us 20.27X Io\ Eosinophilic cationic protein also changed (3.49~tg/L on luw days and 5.39 ~tg/L on hi gh days ); mycloperoxidase also changed (77.39 µ 0 /L on low clays and 138.6 on hi gh ozone days).The authors concluded that "Ozone at ambient concentrations initiates a reversible inl1::11nmatory response of the upper airways in normal children".
Mc Bride ct al (32) recently rerurted on a comparison hetween the nasal inflammatory responses or normal and asthmatic subjects.Exposures usi ng a head dome were to 120 or 240 ppb of ozone or cl ean air for lJO mins with moderate exercise.They reported that the degree or intl ammatmy response, as judged by nasal la vage specimens.was con side rably greater in the as thmatics, thou gh pulmonary !'unction test changes were not different between the two gro ups (and were min imal in degree J.

Effects on the normal lung
Over the pas t 20 years, man y human studies with nllltrollcd exposure to ozone have been reported.These are very briefl y summari zed here.It has been found that: • FYC and forced expiratory volu me in I s (FEY 1) or young normal subjects is lowered by a 6 h exposure with exercise to as little as 0.08 ppm or ozone (the current Canadian l h standard ) (33 ).
• Considerable indi vidual variation in rL•sponse ex ist s in normal subject s, but in a given ind ivid ua l. reprod ucibility is good.Hazucha (34) analyzed resu lts fro m diffe rent laboratories involving a large num ber of no rmal subjects.He made the important observa tion that although one can talk of a 'thresho ld ' va lue for an indivi dual, there is no such th ing as a threshold val ue for a group.Thi s is because.ii' a stati stically signi l'icant shift in mean val ue of a group is considered to be the criterio n, then in one indivi dua l the val ue will have fa llen very significantly as a result of the exposure.This poi nt is ofte n misu nderstood.• The first effect of ozo ne is not to cause airway const riction.but to limi t the maxi mal inspirat ion the indi vidual can take (probabl y through a spina l rctlex activated by irritant rece pto r st imulati on ) {35 ).• T he decrement in lung fu nction cont inues the longer the expos ure, at least over 6 h (33 ).• Over a '+ h period.an ex posu re reg imen starting at 0 ppm, increasing to 0.24 ppm at the max imum and decli ning to O ppm ca uses a greater change in FEY 1 than a constant exposure to 0.1 2 ppm for the whole period (36 ).• When the FYC is lowered, airway infla mmation is demonstrable.Thi s was l'irst shown by Sel tze r ct al (37 ) and Clllll'irmed in add itional studies hy Koren ct al (38) .
A recent re port from Aris ct al (39) invol ved BAL in 12 norma l subj ects I 8 h after expos ure to 0.20 prm ozo ne wi th mode rate exerc ise.There was ev idence or induced inllammat ion as shown by an increase in polymorph, and in intla mmatory med iators.In addition.mo rphometric studies based on hronchi al bi opsy material showed a 6-fold increase in ncutroph il s pcr ~ cm-of tissue fro m the bronchial wall.Koren ct al ( lO ) recently summan zcd the ti me sequence of lhc inllammatory events in normal healt hy subjects fo llowing a 2 h ex posure to 0.4 ppm 01.onL' for 2 h with moderate exerc ise.At the end of the expo~urc, PMNs, prostaglandi n E2 and interleukin-6 were all increased in the BAL lluid and were at higher le vels tha n they were I~ h later.Fibronectin and urokinase-type plas minogen :1ctivator, both of wh ich have been assoc iated with l"ihrogcn ic processes, were at higher levels in BAL al.
1 X h th,m at I h arter ex posu re.Protc' in an<l ti ssue fau o r. wh ic h is J criwd rrom mac rophages.were hoth increased simi larl y at I h and 18 h after exposure .These au thors concl uded: '"It should be noted that desp ite the 'reversibi lity' o r spiromct ri c changes after I ozone J exposure.there is strong evidence ror the prese nce of lung in n,111 1mati on even at 18 h art cr ex posure••.De vlin el a l (4 1 J have compared the change in infl ammatory indi cators after Ji ffen:nt exposures: the ir results arc shown in T nblc 1. In spi te of this strong evidence.there is not a preL•isc rel ationship be tween the runetion decrement induced by ozone an<l the degree o r the infl ammatory response (42.43 ).There is un anim ity that an in fl ammatory response may he found in some indi vi du als with a min imal spirometric response.an<l lesse r degrees of in flammat ion may occ ur in some with a considerab le FYC decrement.T hese observ,1tions rai se di fli cult questions in re lati on to attempt s to set any protec t i vc standa rd.
• Ozone increases lung pe rmeabi lity as incl icate<l by J ict hyl cnctriaminc pentaacct ic ;icid (DTPA ) c learnnn• • The J ecrc mcnt in lun g function continues the longer the ex posure.hut repetitive ex posu res produce less of' a response (thi s 'adaptati on• appears to last f'o r about 10 days ).The most recent st udy of' repetitive ex posures is by Fol in shec an d coll eag ue s (45).Seventeen hea lthy males were exposed to 0.1 2 ppm ozone for 6.6 hon riw con secutive clays .They exerci sed (expired vent ilation 39 Umin) fo r .'iOmins or each hour o r exposure.except fo r a pa use rnr lunch.Com pared with air. the percen tage chan ges in FEY I over the fi ve Jay s were -12.8(;;.-8.7'7o .-2.5%.-0.06% an<l +O.I 8'7r.No increased re spon se occurred on thl' sl.'con<l J ay or cx posurl'.althou gh previ ous J ata had i11J icate<l that th is mi ght t1L-c11r.!\i rway responsiveness was increased after each expos ure.and the ra tios o r methacho line res ponsiveness tn control va lues were 2.22.3.67 .4.5.'i,3.99, 3. 24 an<l 3.74 over the rive da ys or test ing.Th us.there was no attenu at ion or the induced increased airway re sponsiveness with consccutiw expos ures.In one norm al subject.the ozone expClsurc cl i<l not increase airway res po nsiveness .Symptoms o r cough and pain we re prese nt only on the first day.asa l lavage rc ve:.ikd nn increase~ in ncu trophils l.'xcept on the first ozone exposure day .• O;,onc ex posure lim its human maximal athletic perform ance (46).T he mech,rnism of this is unclear becau se it see ms unlike ly to be Jue to an increase in airw ay res istance (47) .Li nder ct al (48) re ported that 07 onc ex posure not on ly limit s max imal oxygen uptake.but alsn shins the an aerobic threshold.T hey st udied 12 men and 12 wo 111cn during bicycle exerc ise until ex haust ion in a climate contro ll ed chamber.Ozone le vels or 0.004.O.Oh to 0.07 and 0. 12 to 0. 13 ppm were use<l.In addit io n to !he chan ge in anaerob ic thresho ld .OLonc caused a ck,ir decrease in performance and in the ti me that maximal work could he sustained.The mec ha nism of these e ffec ts is not precisely delineated : they may ind icate that ventilati on-perfusion mismatch in the lun g has had the effect of lowerin g the arterial oxygen te nsi on si gnifica ntly .No direc t measurements or arterial oxygen tension have been made however.

Effects on asthmatics
Early obse rvations on asthmatics indicated that the percentage change in FEY I fo r a given ozone ex posure was not much different from that in nonasthmati cs.However.when thc<lata arc examined closely.it is clear that such a conclusion leads to an underest imation of the importance of the comparative effects.The recen t observation that the nasa l inflammatory respon se aft er ozone exposure is greater in asth ma tic than in nonasthmatic subjec ts is noted abo ve.
Krei t ct al (-+9) studi ed nine asthmatic and nine normal subj ects (Tabl e 2).Subjects were classified as asthmatic ir they ha<l a history nf re versible L'hest ti ghtness ,uHI wheezing.a previous diagnosis of asthma made by a phy.s ic ian.and a concent ra ti on of methacholin e re4uircd to double baseline specific airway resistance (PC I OOsRaw) of less than 1.5 mg/mL.All of these subjects were fell to haw relati vely rnil<l asthma.Their ages ranged from 21 to .14 yc,irs: rive were women an<l four were men.Rase line FEY I values v,1ricd rrnm 2.5 8 to 5.12 L. The nine normal s were aged J lJ to .11years.and comprised five v,Ol11L'n ,md four men : :ill had normal mcthacholine respon .sivcncss.J\n cxposun~ chamber was used.Exercise was done on a bicycle crgnmeter wi1h minute ventilat ions of about 50 Umin.Ozone concentrations were 0.4 ppm.With initial value., of lung function J() to 4W/r lower in the as thmatics.the effec ts o l a comparahk absolute fa ll in vcntilatory indi ces arc necess,1rily more signit'icanl in terms or li miting exercise capab ility or in causing J ys pnea.
The mcthacho line res ponsiveness c hanged in the normals rrnm a pre-exposure v,llue of .12.l) mg/m l (required to lkiuble airway res istance) tn a po.sto/onc v:illlL' or 8. 5 mg/ml.In the

Ozone in combination with other chemicals
Although initial studies had indicated that ozone and sulphur dioxiJe br athed together mig ht enhance the effect o f each, subscquL•nt ex pe rime nt s failed to suppo rt this conclusion.
Kueni g e t a l (5 I) showed that pre-exposure to ozone enhanced the e ffect of sulphur dioxide in a group of I J adolescent asthmatics .The sequences of exposure studied were: air f"ullowed hy I 00 pph sulphur dioxide; 120 ppb ozo ne f"ol lowed by 120 ppb 01:onc; and 120 ppb ozone fo lloweJ by I 00 ppb sulphur JioxiJc.Air-su lphur dioxide and ozone-o.wnL'exposures did not cause signi licant changes in !"unction.but ozo ne-sulphur dioxiJe exposure caused an 8% fa ll in FEY 1, a 19% increase in respiratory resi stance, and a I YX, decrease in Ymax 50'/r.

CONCLUSIONS
The re i. a wealth o f experimenta l data on the aL:u lL' effects or 01unc on the human subject.The first phenomenon observed is the inducti on o r in!lammati o n.Decre men ts in lung function a re co mplex in o ri g in , anJ may not fo ll ow c lose ly the severity or the inlla mmation induced .T here are many poorly unde rstooJ phenomena, incl uding the precise determinan ts of the incliviclua l re sponse; th e mechani sm of •aJaptation •; the phenomena that underlie the observed reduction in maxima l oxygen uptake; and the consequences o f ozone exposure in co mbinati o n with other irritant s.There is evidence that asthmatic subj ec ts may have a greate r in fl a mmatory response than no nasth mat ics.Although the degree or func ti o n test res po nse is broadly s imilar betwee n no rmal s and asthmatics.the latter may be more severely affected because their initial pulmo nary fun ct io n may be lower, and their pre -exposure degree of airway responsiveness is grtater.

TABLE 1 Percentage change in inflammatory indicators following different exposures of ozone
Can Respir J Vol 2 No 1 Spring 1995 Ozone

TABLE 2 Changes induced by two-hour exposure to 0.4 ppm ozone with intermittent exercise
(30) exposure or 0.12 ppm breat hed for I h at rest was round by Molfi no ct al(50)to lead to a reduction in dose of inhaled allergen fur a g iven respo nse in six or seven as thmatic s.Ir con firmed in larger scale studi es.this woulJ have important long term implic ations .As noted above , Bascom et al(30)did not !ind that u1one exposures enhanced the e ffec t or an allerge n in the nose, although an inlbmmato ry reaction had been induced.