Proc. Instn Civ.Engrs, Transp.,1997, 123,May, 102 – 110

Paper 11337

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102

Criteria for air quality in enclosedcar parksJ. Burnett, BEng(Tech), PhD, CEng, FCIBSE, FHKIE, FIEE, and M. Y. Chan,BEng(Hons), MSEE, AMASHRAE

Written discussioncloses 15 July 1997

J. Burnett,Head and Chair,Department ofBuilding SciencesEngineering,The Hong KongPolytechnicUniversity

M. Y. Chan,Demonstrator,Department ofBuilding SciencesEngineering,The Hong KongPolytechnicUniversity

This paper addresses concerns about theenvironment found within enclosed carparks in Hong Kong. These concerns havebeen expressed in terms of the levels ofcarbon monoxide found in car parks fol-lowing a survey. The outcome has been thedevelopment of a code of practice for carpark ventilation system design and per-formance. It is recognized, however, thattoo stringent criteria may lead to over-design of ventilation systems and unwar-ranted use of energy. The criteria need totake into account the levels of exposurelikely to be experienced by two separategroups – car park users and car parkemployees or maintenance contractors. Inpublic car parks the former groupembraces the entire population as driversor passengers, whilst the latter group canbe expected to include only working adults.The criteria for exposure for each groupare considered with respect to varioushealth and occupational guidelines used indeveloped countries. In addition, thepotential for impairment of driving per-formance is considered. Criteria for safeexposure levels in car parks arerecommended.

IntroductionIn a built-up metropolis such as Hong Kongthere is great demand for but short supply ofland and building space. This is manifest invery high prices and rents for commercial,industrial and residential property. Commer-cial and industrial buildings are almost invari-ably high-rise and densely populated. Thetrappings of an affluent and industrious societyincludes a relatively large vehicle populationfor the usable land area. The 270 000 or soprivate cars comprise 59% of all registeredvehicles. There is a big demand for off-streetparking. Large multistorey underground carparks are common solutions for both thegovernment and developers (Fig. 1). Increasedaffluence and wider education are also makingthe people of Hong Kong more aware ofenvironmental issues, not least air quality.Enactment of new legislation governing air

quality1 and vehicle emissions,2 and strength-ened environmental policies3 have requiredresponses from commerce and industry.

2. A report by the Consumer Council4

highlighted the perceived poor quality of theenvironment within public car parks and statedthat many did not meet World Health Organ-ization (WHO) ‘standards’ for carbon mon-oxide (CO) concentrations in the air. Thisconclusion was based on CO measurements in37 car parks undertaken by the EnvironmentalProtection Department (EPD) during the sum-mer of 1992. The report concluded that morethan half of the car parks surveyed had unac-ceptable air quality in terms of CO level: thatis, above 100 p.p.m. (115 mg/m3) for 5 min.The results are summarized in Table 1. Thereport mentioned that legislative control of theair quality in car parks was under considera-tion by the government.

3. Subsequently the EPD issued a con-sultative document5 in which the prescribedmaximum level for CO is an average of 100p.p.m. over 5 min. This compares with airquality design criteria for car parks used hith-erto, such as those of the American Society ofHeating, Ventilation and Air-conditioningEngineers (ASHRAE)6 (maintained level of 50p.p.m., peak not to exceed 125 p.p.m.) and theInstitution of Structural Engineers7 (50 p.p.m.normal, 100 p.p.m. peak, 250 p.p.m. atentrance and exit tunnels). Hong Kong’sEPD’s consultative document was recentlypublished as a ‘practice note’.

4. Existing car parks may fall within theproposed legislative controls. They may havebeen designed using air change rate criteria,either on the basis of air changes per hour,6,8

air flow per unit area,6, 9, 10 air flow per parkingspace,11 or air flow per operating engine.10, 11

Given the local preference for ASHRAE’sguidelines, a target ventilation rate of six airchanges per hour is likely to be the most com-mon approach. With car park floor heightsnormally less than 3 m, the alternative recom-mended9 ventilation rate of 7·5 l/s/m2 wouldprovide more than six air changes per hour.Whilst these design approaches have beenfound to be effective in maintaining air qualityin car parks elsewhere,12, 13 apparently theymay not suffice for heavily used car parks inHong Kong, even if the system is properly

Table 1. EPD results and air quality guidelines and standards for CO

EPDresults4

CO concentration for various timeintervals: p.p.m. (mg/m3)

Notes

Ceiling 5 min 15 min 1 h 8 h

Maximum 330(380)

230(264)

161(186)

Excludes one car park wherethe ventilation system was notrunning

Average 115(133)

71(82)

50(57)

As above

Minimum 34(39)

8(9)

3(4)

As above

HKAQO1 27(30)

9(10)

Protection of all members ofsociety

WHO18 87(100)

25(29)

10(11·5)

To prevent COHb levelsexceeding 2·5–3% in non-smoking populations based onCoburn formulae

WHO22 100(115)

25(29)

10(11·5)

ACIGH26 # 25 (29) TLV-TWA. #Previous STELreplaced by BEI of 8% CAHb

NIOSH27 200(229)

35(40)

Health effects: cardiovasculareffects. For documentscontaining NIOSHrecommendations for safetyand health of carbonmonoxide, see references 4and 7

OSHA28 125(144)

50(55)

This is a standard forindustrial workplaces16

HSE24 300(330)

50(55)

Fig. 1. Interior of apublic car park

AIR QUALITY INENCLOSED CAR PARKS

designed, commissioned and maintained.Alternative design approaches use either asimple dilution equation based on averagevehicle emission data and assuming perfectmixing,7, 11 or a dilution model that includesvehicle operation and emission data.10 This lat-ter approach is adopted in the EPD’s practicenotes.5

Assessment of exposure risk5. The passage of laws to protect public

health and the environment has brought intofocus the need to assess both the risk and theeconomic impact. Risk assessment is complic-ated because individuals and organizationsperceive risk dif ferently, depending on who isperceived to be ‘responsible’ and who pays forany ‘clean-up’. For car parks used by the gen-eral public, benefits and responsibilities argu-ably accrue to both patrons and owners. Riskassessments associated with air quality willtake account of dose and consequent healtheffects. Exposure assessment is the determina-tion or estimation of the magnitude, frequency,duration and route of exposure. Health effectsmay be characterized by individual lifetimerisk, relative risk between exposed and unex-posed persons, and temporary debilitatingeffects. Some of the key points that need to beconsidered in risk assessment are14

(a) the uncertainties in estimating the extentof health effects

(b) the most appropriate dose-response andexposure assessments

(c) which population groups should be theprimary target for protection.

6. There is a great deal of data available onthe effects of exposure to CO, including phys-ical impairment and health risks due to short-term and long-term exposures, but it has notresulted in uniform agreement on exposurecriteria amongst various agencies worldwide(Table 1).

7. The prescribed criteria for air quality,based on CO concentration, will be critical inthe design and operation of car park and theirventilation systems and will impact on energyconsumption. It is important that the criteriatake into account a reasonable level of risk toboth patrons and car-park employees. Endeav-ours to achieve unnecessarily low CO concen-trations may cause undue capital investmentand unwarranted use of electrical energy.

The environment within enclosed carparks

8. Concern about the environment withinenclosed car parks embraces fire safety, ther-mal comfort and air quality. Employees and

subcontractors providing security, mainten-ance and other services may regularly work ina car park for periods of up to 12 h. They maywork in one location or move around a largearea. For them the health risk is generallyfrom long-term exposure to pollutants, andoccupational health criteria would be applic-able. Car park patrons, including regular dailyusers and irregular visitors, tend to occupy a

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car park for no more than 10 min, usuallymuch less. For them it is discomfort andshort-term effects from exposure that are ofconcern. For all drivers it is necessary to con-sider the impact of exposure on driving per-formance, lest this be a factor determining thesafe levels of pollution.

9. The feeling of wellbeing or otherwise forpeople in car parks depends on a number offactors. With inside temperatures 1–8°Chigher than those outside,4 the summer tem-peratures of 30°C or more coupled with highhumidity means the thermal comfort withinenclosed car parks in Hong Kong can be mostunsatisfactory. This discomfort can contributeto negative reactions to pollutants and odours.In extremes of thermal discomfort and highlevels of pollution, sensitive people may findthe conditions overwhelming.

10. The pollutants found in enclosed carparks are mainly due to vehicle emissions.There is a distinct dif ference between petrol-and diesel-engined vehicles, and the amount ofpollutants discharged depends very much onthe particular vehicle and the mode of opera-tion. Williams15 has summarized the contribu-tion of motor vehicles to air pollution,distinguishing between primary pollutantswhich may affect people in the form in whichthey are emitted, and secondary pollutantswhich undergo or are formed by reactions inthe atmosphere.

11. Petrol and diesel produce oxides of car-bon and hydrogen as a consequence of incom-plete combustion. Of the most concern are theprimary pollutants: CO, carbon dioxide (CO2),nitrogen oxide (NO), nitrogen dioxide (NO2),sulphur dioxide (SO2), smoke and particulatematerial, lead and the range of organic com-pounds known generally as volatile organiccompounds (VOCs). VOC emissions ariselargely from evaporative and futile lossesrather than from combustion. Of these pollut-ants, CO, NOx( 5 NO 1 NO2), SO2 and leadcompounds are toxic. NO2 is the most toxicsubstance but typically accounts for around 5%of the total NOx emitted.16 Total hydrocarbons(THC) include polynuclear (or polycyclic) aro-matic hydrocarbons (PAH), a group of severalhundred organic compounds, the best knownof which is benzo[a]pyrene (BAP), a suspectedhuman carcinogen.

12. Although catalytic converters are usedextensively, because of the low sulphur con-tent of petrols supplied in Hong Kong, hydro-gen sulphide (H2S) emissions are not regardedas significant in the context of health impacts.

13. Petrol engine NOx emissions are smallat idle and low speeds, and increase roughly inproportion with speed. Similar patternsemerge for THC emissions. CO emissions aregenerally highest at idle and very low speeds(0–20 k.p.h.)16 For petrol-engined vehicles at

idle and low speeds, CO emissions are typic-ally an order of magnitude greater than NOxemissions, and given that permitted concentra-tions are of a similar magnitude, it is the COlevel that dominates vehicle exhaust dilutionrequirements. Ball and Campbell17 have illus-trated that dilution rates required for petrolengines in various conditions were dominatedby the need to dilute CO, whereas NOx domin-ated for diesels, but at much lower rates. InHong Kong, petrol-engined cars dominate,with diesels accounting for less than 1% of thetotal. If car park CO is within safe levels it isgenerally accepted that all other contaminantswill also be within safe levels.

CO inhalation14. CO is a colourless, odourless, tasteless

and non-irritant gas, slightly lighter than air,which is mainly encountered as a product ofincomplete combustion. CO’s primary toxicaction is the inhibition of cell oxidation follow-ing inhalation exposure, mainifest in increasedcarboxyhaemoglobin (COHb) levels in theblood. Hypoxia caused by CO leads to defi-cient function in sensitive organs and tissuesof the heart and brain. At COHb levels below10% it is mainly cardiovascular and neuro-behavioural effects that are of concern.Cardiovascular effects may have health impli-cations for the general population in terms ofcurtailment of certain physically demandingactivities. High risk groups include the elderly,the very young, pregnant women and, in par-ticular, chronic angina patients.18 The absorp-tion and elimination of CO is estimated usingthe widely accepted mathematical model ofCoburn et al.19 The most influential factors indetermining COHb levels are CO concentra-tion, duration of exposure and alveolar ventila-tion, which is dependent on work rate.

15. The risks associated with exposure toelevated levels of CO also need to takeaccount of everyday exposures. The primarysource of CO in the urban environment ismotor vehicles. Ambient air concentrations inurban areas depend on the weather and trafficdensity and vary greatly according to time anddistance from the source. Primary air stand-ards are intended to protect the population atlarge and take into consideration daily expos-ure of the very young, the very old and theseriously ill. Hong Kong’s Air Quality Object-ives (HKAQO)1 set targets for outside ambientair in Hong Kong that are similar to the pri-mary air standards of the US.20 The prescribedlevels for CO (Table 1) are intended to protectagainst the occurrence of COHb levels above2%.21 These levels are rarely exceeded atstreet-side monitoring stations in Hong Kong.Readings are generally around a few parts per

Table 2. Guidelines for exposure conditions that would prevent COHb levelsexceeding 5% in non-smoking populations (WHO22)

CO concentration Exposure time notto be exceeded

Concentration thatwould produce 5%

COHb‡

Safety factor

p.p.m. mg/m3 Lightwork*

Heavywork†

Lightwork

Heavywork

Lightwork

Heavywork

200§100§

75503525

230115

86554029

15 min30 min60 min90 min

4 h8 h

—15 min30 min60 min

2 h8 h

298157

87643731

—193105

624130

1·51·61·21·31·11·2

—1·91·41·21·21·2

* Light work 5 181 l/min1

† Heavy work 5 30 l/min1

‡ Calculated using Coburn’s equation§ Maximum permissible concentration

Table 3. Predicted COHb levels for subjects engaged indifferent types of work (WHO22)

COconcentration:p.p.m.

Exposuretime not

Predicted COHb level forthose engaged in

Sedentarywork

Lightwork

Heavywork

100502510

15 min30 min

1 h8 h

1·21·11·11·5

2·01·91·71·7

2·82·62·21·7

Table 4. CO exposure duration required to reach3·5% COHb (ACGIH23, 26)

CO concentration:p.p.m.

Time: min

Sedentarywork

Lightwork

Heavywork

1000500300200150100

7550

182434465886

117191

10131824314662

102

811152127395387

Alveolar ventilation:l/min

6 15 20

AIR QUALITY INENCLOSED CAR PARKS

million,1 although peaks of the order of 20p.p.m. are found on occasions.

16. CO is widely generated indoors due toinadequate ventilation of combustion appli-ances. Short-term concentrations of 10–30p.p.m. have been measured in German kit-chens, and values of 50 p.p.m. and higher werefound in 17% of homes in Dutch cities. Entrain-ment from integral or attached garages intoliving and working spaces can cause excessiveCO levels. Levels found in workplaces, such asin car repair shops, may exceed 500 p.p.m.18

Tobacco smoking is also an important sourceof CO indoors, and this can be the largestconstituent of CO intake in those who do notsmoke. As a consequence, healthy personshave endogenous COHb levels of 0·5–1%. Theaverage COHb level in the general populationof non-smokers is 1·2–1·5%, in cigarette smok-ers around 3–4%, but for heavy smokers it canexceed 10%.18

Guidelines for occupational exposure17. The aforementioned Consumer Council

report4 includes figures on ‘occupationalexposure limits’ for non-smokers, which wereabstracted from an early WHO publication22

and are reproduced here in modified form asTable 2. These guidelines would preventCOHb levels exceeding 5% in non-smokingoccupational groups. The given safety factorsare obtained by dividing the concentrations incolumns 5 and 6 by those in column 1. Unlessotherwise indicated, the guidelines given inTable 2 ‘. . . should be considered as desirablerather than maximum acceptable limits’.22

18. The WHO guidelines on exposure thatwould prevent COHb levels exceeding 2·5–3%in general non-smoking populations are givenin Table 1 using two dif ferent 15-min limits:100 p.p.m.22 and 100 mg/m3 (87 p.p.m.).18

Table 3 gives an estimate of COHb uptakeunder dif ferent types of work. Table 4 showapproximate times, calculated from Coburn’sequation, at various exposures to reach aCOHb level of 3·5% where there is no otherexposure to CO during the day.23 These expos-ures are somewhat longer than those of Table2, even when the safety factors are taken intoaccount. The data given in these tables showthat there is a measure of inconsistencybetween CO dose and estimated COHb uptake.

19. The American Conference of Govern-ment Industrial Hygienists (ACGIH) has influ-enced occupational exposure standardsworldwide, including those of the UK’s Healthand Safety Executive (HSE)24 and HongKong’s Labour Department.25 ACGIH thresh-old limit values (TLVs) ‘. . . refer to airborneconcentrations of subtances and representconditions under which it is believed that

nearly all workers may be repeatedly exposedday after day without adverse effect. Becauseof wide variation in individual susceptibility,however, a small percentage of workers mayexperience discomfort from some substancesat concentrations at or below the thresholdlimit; a smaller percentage may be affected

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more seriously by aggravation of a pre-existingcondition or by development of an occupa-tional illness’.

20. Two categories of TLVs are specified.The TLV-TWA is the time-weighted averageconcentration for a normal 8-h workday and a40-h work week to which nearly all workersmay be repeatedly exposed, day after day,without adverse effect (see appendix). For COthis was, up to 1992, 50 p.p.m., but has sincebeen reduced to 25 p.p.m.23 Until recently theshort-term exposure limit (TLV-STEL) for COwas 400 p.p.m.,23 this being ‘. . . the maximalconcentration to which workers can beexposed for a period up to 15 minutes con-tinuously without suffering from 1) irritation,2) chronic or irreversible tissue change, or 3)narcosis of sufficient degree to increase acci-dent proneness, impair self-rescue, or materi-ally reduce work efficiency, provided that nomore than four excursions per day are per-mitted, with at least 60 minutes between expo-sure periods, and provided that the dailyTLV-TWA also is not exceeded’.26 The TLV-STEL should be considered a maximal allow-able concentration, or ceiling, not to beexceeded at any time during the 15-min excur-sion period. The TLV-STEL has since beenreplaced by a biological exposure index (BEI).

21. Biological monitoring consists of anassessment of overall exposure to pollutantsthrough measurement of the appropriate deter-minant(s) in biological specimens collectedfrom a worker at a specified time. BEIs repres-ent the levels of determinants which are mostlikely to be observed in specimens collectedfrom a healthy worker who has been exposedto a pollutant to the same extent as a workerwith inhalation exposure to the TLV of thatpollutant. BEIs do not indicate a sharp distinc-tion between hazardous and non-hazardousexposures, and it is possible for an individualto exceed the BEI without incurring increasedhealth risk. For CO the determinants areCOHb in blood, for which the BEI is ‘less than8% haemoglobin’, and CO in end-exhaled air,for which the BEI is ‘less than 40 p.p.m.’.Samples are taken at the ‘end of shift’. It isnoted that some workers with increased sus-ceptibility to the effect of CO are left unpro-tected by the recommended BEI.

Occupational safety standards22. The US National Institute for Occupa-

tional Safety and Health (NIOSH) providesrecommendations for safety and health stand-ards in the workplace27 which are transmittedto the Occupational Safety and Health Admin-istration (OSHA) of the US Department of

Labor for use in legal standards.28 NIOSHrecommended CO limits (Table 1) are basedon cardiovascular and behavioural evidence:29

‘. . . The recommended TWA standard of 35p.p.m. CO is based on a COHb level of 5 percent, which is the amount of COHb that anemployee engaged in sedentary activity wouldbe expected to approach in eight hours duringcontinuous exposure. The ceiling concentra-tion of 200 p.p.m. is based upon the restrictionof employee exposure to CO to transientexcursions above 35 p.p.m. which would notexpect to significantly alter his level of COHb.’The recommended standard does not take intoconsideration the smoking habits of theworker.

23. The HSE published limits24 form part ofthe requirements of the Control of SubstancesHazardous to Health (COSHH) Regulations.The HSE’s occupational exposure standard(OES) is the concentration of an airborne sub-stance, averaged over a reference period, atwhich, according to current knowledge, thereis no evidence that it is likely to be injuriousto employees if they are exposed by inhalation,day after day, to that concentration. The cur-rent OES for CO is 50 p.p.m. 8-h TWA, and 300p.p.m. 15-min STEL.

24. It is apparent from this review that theprimary health concern for workers is theCOHb uptake which, in turn, determines limitsof CO dosage (the integral of exposure overtime). The long-standing 8-h TWA of 50 p.p.m.is based on an estimated 5% COHb uptake,although NIOSH argues that 8-h exposure to35 p.p.m. achieves this level. The more recentACGIH TLV of 25 p.p.m. is ‘. . . recommendedto retain blood COHb levels below 3·5%, toprevent adverse neurobehavioural changes,and to maintain cardiovascular exercisecapacity’.20 The reason is the interpretation ofdata which ‘indicate that COHb levels as highas 5% identified for OSHA permissible expo-sure limits (PELs) and NIOSH recommendedlimits (RLs) place workers with cardiovascularand respiratory disease, those who are preg-nant, or those with the need to performpsychomotor tasks at higher risk of adverseeffects’.23

25. Studies on the impact of CO exposureon driving performance have produced incon-sistent results. Wright et al.30 reported a sig-nificant deficit in ‘careful driving’ habit, butstatistically insignificant facilitation of emer-gency type actions with a 3·4% increase ofCOHb level. On the other hand, McFarland31

reported that 6% COHb had no effect on driv-ing ability. City driving itself can lead to sig-nificant CO exposure. If 3·5% COHb uptake istaken as a limit, a STEL of 200 p.p.m. is sug-gested. The data given in Table 4 suggest that

Table 5. Summary of CO levels measured in various car parks in Hong Kong

Physical size Total number 5-min TWA: p.p.m. 15-min TWA: p.p.m. 1-h TWA: p.p.m.

Minimum Maximum Minimum Maximum Minimum Maximum

Under 3000 m3 12 13 156 8 92 6 61·43000–10 000 m3 15 15·4 115 12 51·6 8·2 34·510 000–30 000 m3 14 6·4 506·6 3 361·6 2 153·8Over 30 000 m3 11 5·5 256·8 5·3 177 5 129

Fig. 2. Narrow rampsand sharp corners asfound in many HongKong car parks

AIR QUALITY INENCLOSED CAR PARKS

this would leave ample allowance for CO expo-sure associated with city driving.

Exposure limits for car parks26. It is evident from Table 5 that CO lev-

els in underground car parks vary consider-ably. Not only is there considerable variationbetween car parks but also within a given carpark. Measurements taken by the author showvariation between 10 and 260 p.p.m. in oneparticular large car park, with the heaviestconcentration adjacent to the ramps (Fig. 2),as is generally to be expected.

27. Notwithstanding that the ACGIH’sguidelines are intended for use in the practiceof industrial hygiene and should be interpretedand applied as such, they do offer an indica-tion of risk from exposure to CO in car parks.However, it should also be remembered thatall these occupational guidelines have beenestablished for single chemicals and do nottake into account the effects of exposure tomixtures.

28. Car park management can controlworkers’ exposure by implementing workingpractices that maintain shift exposures withinthe 8-h TWA. Fee collectors and the like canbe housed in pressurized rooms, and securitystaff trained to avoid long periods in areaswhere high concentrations are expected. Ifexposed workers are not drawn from vulner-able groups, then the 8-h TWA of 35 p.p.m.(OSHA) or 50 p.p.m. (HSE) would be appro-priate. Imposing the lower limit would reducethe working time for exposures to elevatedlevels.

29. The 15-min STEL is relevant in that itgives an indication of the levels to which usersmay be exposed without serious harmfuleffects. According to ACGIH, exposure to 200p.p.m. for 15 min for a smaller adult engagedin light work could produce a COHb level of5%. According to WHO,18 if the 1-h mean valueof 25 p.p.m. is taken as a short-term guidelinevalue, then very high CO levels in the aircould occur for shorter exposure times; forexample, a 5-min exposure could reach 300p.p.m.

30. From Table 4 it can be concluded thatcar park users carrying objects to and from avehicle (light work) and exposed to 100 p.p.m.

for a duration of 15 min are subject to a verylow health risk. This would allow exposure upto 200 p.p.m. for durations of around 5 minwith little apparent effect. At these levels it isalso unlikely that driving performance will beaffected.

31. The actual CO dose will depend on COlevels in the breathing zone. This may be esti-mated from the routes followed by users inleaving and returning to their vehicles. Well-planned pedestrian routing coupled with goodtraffic management (control of vehicular flow,good directional signs, reduced queuing, etc.)can significantly impact on CO dose, whichmay be well below that estimated from the COlevels prevailing in the car park. That is to say,breathing zone exposures could be much lessthan design control limits.

Pollutant load32. Many factors influence the CO level at

a given location within a car park. To make areasonable estimate of the pollutant load foruse in the dilution-based design approach5, 10

valid data on both vehicle emissions and trafficvolume and flow patterns are required, other-wise ventilation rates may be impracticallyhigh.

33. CO emission from a particular vehiclewill depend on engine size, condition (tuning)

107

Fig. 3. Ventilationsystems: (a) forceddisplacement; and (b)conventional mixing

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BURNETT ANDCHAN

and operation, that is on whether the vehicleis travelling at or near car park limit speed(typically 8 k.p.h.), idling or accelerating.16

Cold emissions from a vehicle leaving the carpark are generally greater than hot emissionswhilst entering to park.16 Comprehensivevehicle exhaust emission data are not readilyavailable for vehicles used in Hong Kong. USEnvironmental Protection Agency (EPA) dataare available,32 but as a result of EPA controlsthere has been a significant reduction in COdischarged from vehicles in recent years,although test data on vehicles indicates that,generally, actual vehicle emissions exceed EPAstandards.33 The use of historic ‘rule of thumb’data17, 34 can lead to considerableoverestimates.

34. In heavily used car parks with hourlyparking, traffic volume can be relatively high,with strong CO ‘line sources’ generated whenthere is congestion and queues form. Usersoften tend to seek parking close to exits,which can increase cruising distance andidling times. Traffic management is anotherfactor in determining CO load. Even with reli-able data on vehicle type and turnover, giventhe variables in cruising and idling times, esti-mating the CO ‘load’ in a large car park is notan easy matter.

Car park ventilation systems35. The dispersal of high concentrations by

removal or dilution will be dependent on theeffectiveness of the ventilation system.Mechanical ventilation systems for enclosedcar parks can be classified as

(a) supply-only systems(b) exhaust-only systems(c) combined supply and exhaust systems.

36. Combined systems can provide controlof both supply and exhaust functions. Suchsystems allow more control of the distributionof intake (outside) air and, by adjustment andbalancing, can prevent uncontrolled migrationof polluted air to other parts of the car park.Car park ventilation systems can also be classi-fied according to air flow pattern as unidirec-tional (displacement) systems – in which thesupply and exhaust locations are arranged toestablish an overall flow through a significantvolume of the car park, and multidirectional(mixing) systems – in which supply andexhaust points are arranged so that a numberof relatively small flow fields are establishedover relatively short distances. Fig. 3 illus-trates the two types of system.

37. Properly designed and maintained uni-directional systems can provide a sweepingaction which moves pollutants quickly to theexhaust vents. A disadvantage of such sys-tems, particularly in large area car parks, is

that high pollutant gradients can be estab-lished, as the supply air is contaminatedsequentially and cumulatively as it flowstowards the exhaust side. This can result inhigh pollutant levels in areas near the exhaustvents. High total air flow and low infiltrationfrom adjacent areas are required if effectiveventilation is to be achieved.34

38. Multidirectional systems providesources of outside air for the dilution of pollut-ants, and when properly designed and oper-ated can provide for more ‘localized’ control ofpollutants. However, such systems increasethe likelihood of short circuits in the ventila-tion flow. For example, if supply and exhaustgrills are too closely coupled, air can flow fromthe supply grill to the exhaust grill withoutadequate mixing with the pollutant-laden air inthe region of the flow path. This can result ina significant volume of air being moved, butlimited impact on overall pollutant loading.Short circuits may be exacerbated by obstruc-tion and parked vehicles.35 External short cir-cuits can also occur when intake and exhaustports on the exterior of the car park structureare too close, or are coupled by adverseenvironmental conditions, particularly wind.36

Studies have suggested that displacement ven-tilation is superior to mixing ventilation as faras ventilation effectiveness is concerned.35

39. Short circuit ventilation can increaseventilation system efficiency if the resultingflow pattern is through the regions of highestpollutant source, as it removes the pollutants

AIR QUALITY INENCLOSED CAR PARKS

before they have had time to mix throughoutthe volume of car park. However, too muchreliance on ‘localized ventilation’ can lead toinefficient removal of pollutants once they areoutside the ventilation zone of the localizedventilation system. Adequate flow must bemaintained in all parts of the car park. InHong Kong the combined supply and exhaustsystem is favoured, with the followingconsiderations(a) avoiding long flow fields that permit the

contaminant levels to build up above anacceptable level at the end of the flowfield

(b) providing short flow fields in areas of highpollutant emission

(c) providing an efficient, adequate flowthroughout the volume of the parkingstructure

(d) avoiding stratification of engine exhaustgases

(e) the contaminant level of the outside airdrawn in for ventilation (it is possible thatthe ‘fresh air’ intake into car parks will bepolluted at or above the HKAQO1 level)

(f) the thermal comfort of occupants.40. It should be noted that the guidelines

of 7·5 l/s may work only if a reasonable evenmixing of the air flow and the contaminantsource is achieved. Otherwise localized prob-lems will exist, even if a higher ventilation rateis provided. From the standpoint of efficientextraction, exhaust points from both high andlow levels are preferred, but this is seldomprovided in Hong Kong installations.

Ventilation system control41. The monitoring of CO concentrations in

the environment of a car park is not a straight-forward matter. There are several basic meth-ods used in a variety of commercialequipment. Most of this equipment is nothighly accurate. At levels in the 10–100 p.p.m.range, overall accuracy may be no better than6 10 p.p.m., even when the equipment is prop-erly zeroed, ranged and calibrated.37

42. Readings of CO concentration overtime will depend on sensor response and log-ging frequency. Recording transient variations,occurring when cars pass by or engines startnearby, and obtaining TWA values will dependon the instrument used. It is conceivable thatdif ferent instruments with dif ferent operatorswill measure dif ferent TWA values for thesame location and time duration. Clearly, whensetting strict air quality standards it is neces-sary to define the type of instrument to beused, the method to be used, the locations tobe sampled and the sampling time as part ofthe standard.

43. In order to avoid unnecessary use ofenergy, ventilation fans should operate only

when unacceptable CO levels arise. It will beprudent to install a control system activatedeither by sensors or by time-switching control,with settings based on operating experience.Energy savings can significantly affect the costof the ventilation system. The choice and loca-tion of a CO sensor will be particularly critical.They are not necessarily expensive, but dueconsideration must be given to service life,reliability, operating checks, calibration, clean-ing, etc. Accuracy at around 50 p.p.m. may begreater than 6 20%.

Conclusions44. Given that higher level transient excur-

sions are not necessarily very risky, it shouldbe possible to allow levels up to around 200p.p.m. for durations up to 15 min. This can beachieved with well-planned pedestrian routing,good traffic management, and possibly local-ized ventilation for areas with high pollutionsources, such as exit ramps. It would not beprudent to allow high CO levels to persist onaccount of possible entrainment to other partsof the building.

45. It is also clear that present designmethods for car parks need to be evaluatedagainst actual installations. More data arerequired on pollutant sources and estimationsof pollutant load. Consistent measurement ofCO levels is not easy, neither is measurementof ventilation effectiveness in large volumessuch as car parks. Further field studies arerequired.

46. The recommended CO level proposedby the EPD5 is far too conservative as far ashealth risk to the general population is con-cerned. The car parks surveyed by the EPDare not so dangerous to health as implied inthe Consumer Council report.4

Acknowledgements47. The author would like to thank Dr Wei-

zen Liu for her assistance in reviewing thepaper. The paper is part of a study on car parkventilation funded by The Hong Kong Poly-technic University.

Appendix. Time-weighted average (TWA)48. The term ‘8-h reference period’ relates

to the procedure whereby the occupationalexposures in any 24-h period are treated asequivalent to a single uniform exposure for 8 h(the 8-h TWA exposure).24 The 8-h TWA maybe represented mathematically by

(C1T1 1 C2T2 1 . . . . . . 1 CnTn)/8

where C1 is the occupational exposure and T1

is the associated exposure time in hours inany 24-h period.

49. The short-term reference periodadopted by ACGIH, OSHA, HSE, etc., is 15

109

110

BURNETT ANDCHAN

min. Exposure should be recorded as the aver-age over the specified short-term referenceperiod and should normally be determined bysampling over that period. Where the expos-ure period is less than 15 min the samplingresult should be averaged over 15 min. Anexample quoted by HSE24 is a 5-min sampleproducing a level of 600 p.p.m. immediatelyfollowed by a period of zero exposure. The15-min average exposure would then be 200p.p.m.

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