Air Pollutants in Road Tunnels

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ELSEVIER The Scienceof the Total Environment 189/190 1996) 431-435

Air pollutants in road tunnels

AbstractConcentrations of specific vehicle-emitted volat ile hydrocarbons, including 1,fbutadiene and benzene, weredetermined in two urban road tunnels in Giiteborg, Sweden. Comparisons were made with other polluted environ-ments. Samples were taken on adsorbent cartridges and the hydrocarbons were analyzed by gas chromatography. Theconcentrations were several times higher in the tunnels than in streets with similar traffic. The exposure levels ofdrivers and passengers inside cars differ similarly. The resulting dangers for asthmatic children and adults, and theincreased genotoxic hazards for all motorists in tunnels am of particular concern. It is concluded that long roadtunnels with large numbers of cars must be questioned with regard to health hazards. Motorists have reason to avoidtunnel driving and to oppose new road tunnels, Nevertheless, several bil lion-dollar investments in new tunnels areplanned in Scandianvia and Europe.

Keywords: Road tunnels; Human exposure; Volat ile hydrocarbons; Benzene; Butadiene

1. IntroductionEfforts to support the present road transporta-

tion system and to find space for the rapidlygrowing fleet of vehicles have led to extensiveplans for new road tunnels in Scandinavia andEurope. A major concern with road tunnels is theelevated level of human exposure to air pollu-tants, when driving through a road tunnel. Spe-cific inorganic and organic pollutants have beenstudied in tunnels in Sweden [l], Belgium [Z] andGermany [3]. American tunnel studies demon-strate that three-way catalysts do not reduce pri-vate car emissions and air pollution levels ase~ciently as anticipated [4].

The increased knowledge, in recent years, ofhuman health hazards due to air pollutants [5],necessitates a lowered human exposure. Today,non-smokers get a large prounion of their doseof air pollutants in environments polluted bytraffic. This article discusses road tunnels in termsof air pollutants and health hazards.2. Road tunnels planned in Sweden

At present, billion-dollar investments in newtransportation systems are being considered forthe Stockholm region, for the Giiteborg region,and for a road link across the bresund channelbetween Sweden and Denmark.

0048-9697/96/%15.00 1996 Elsevier ScienceB.V. All rights reservedPII SOO48-9697(96)05242-4


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432 G. Barrefors / The Science of the Tota l Environment 189/190 (1996) 431-435Table 1Concentrations (fig/m) of hazardous hydrocarbons in road tunnels as compared with a smoky cafe and a private car

CH, C-C,)EthenePropene1,3-ButadieneBenzeneToluene

Tingstad tunnel Gnistlng tunnelb Smoky cafe Car couped Rural air Giiteborg3600 2250 570 630280 160 42 30 0.5100 65 37 15 0.225 17 12 1.3 0.0330 194 38 55 0.9630 365 40 110 0.6

a Sampling near tunnel exit: 19 February 1992, 08.10-08.40, -4C 3200 vehicles per hour, O-70 km/h, 10% heavy-duty trucks [ l].b 5 March 1992, 08.00-08.20, +4OC, 3200 vehicles hour, 50-70 km/h, 10% heavy-duty trucks.er 15 April 1992, 13.25-13.55, Junggrens Cafe, Giiteborg [6] .d 24 September 1992, 07.50-08.10, + 14C, Central Giiteborg, O-60 km/hour [6] .

In Stockholm, a planned circumferential routewould include about 13 km of road tunnels. Anexternal transversal road with about 9 km intunnels is also considered. In Goteborg, threetunnels of 8 km total length are planned. Theplanned 20 km combined road and railway con-nection over the oresund channel also includes a4 km long tunnel.

3. Air pollutants in road tunnels3.1. Different types of tunnels

Tunnel ventilation can be either transverse orlongitudinal. Lower costs for construction andservice are reasons why longitudinal ventilation isnormally preferred. The longitudinal ventilation iscaused mainly by the piston action of the traffic inone-way tubes. Additional mechanical ventilationis necessary, especially during traffic congestions.The vehicle emissions accumulate along a roadtunnel, and give rise to the highest levels of airpollutants at the tunnel exit. With the same trafficintensity, the maximum concentrations increasewith the length of the tunnel. A method to avoidextremely high concentrations in long road tun-nels is to construct ventilation towers, with fansfor supply air and vented air. The idea is topartially replace the air in the tunnel and to dilutethe remaining polluted air. Techniques to cleanthe particles from polluted tunnel air exist, whileremoval of gaseous pollutants does not seem to berealistic today.

3.2. Concentration levelsConcentrations of a wide range of traffic-emit-

ted volatile hydrocarbons were recently reportedfor the Tingstad tunnel in Goteborg with one-waytubes [l]. Samples were taken on adsorbent car-tridges and analyzed in the laboratory using ther-mal desorption and gas chromatography [6].In Table 1, representative concentration levelsfor ethene, propene, 1,3-butadiene, benzene andtoluene (methylbenzene) are given for four differ-ent severely polluted urban environments. In roadtunnels, cars and outdoor urban air, these hydro-carbons normally originate predominantly fromemissions of petrol-fuelled vehicles. Their concen-trations are then indicative of the level of othertraffic-emitted air pollutants in urban air as well.The first two samples give rush hour levels intwo different urban road tunnels in Giiteborg.The 500 m long Tingstad tunnel consists of twoone-way tubes. The 700 m long Gnistang tunnelcarries two-way traffic. The next two samplesrepresent high indoor levels of cigarette smoke,and rush-hour levels of vehicle-emitted pollutantsinside a car during urban driving. The last columngives background pollution levels.Although the tunnels studied are short, 500-700 m, the pollution levels are several times higherthan inside a car during urban driving. The con-centrations of hydrocarbons from cigarette smokein the cafe are similar to those inside the car fromtraffic emissions. Compared with the backgroundlevels in rural air, the concentrations in the roadtunnels are more than five hundred times higher.


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G. Barrrfors / The Science of the Total Environment 189/190 (1996) 431-435 433The flow of air through the two tunnels studied

is governed by the piston action of the traffic indifferent ways. In spite of poorer ventilation be-cause of two-way traff ic and the same amount ofvehicles, the top concentrations of hydrocarbonswere often lower in the Gnistang than in theTingstad tunnel. This may be due to the muchlarger air volume in the wider and higherGnistang tunnel. Pulsating traffic, due to trafficlights, also improves the ventilation in theGnistang tunnel.In vehicle exhaust, the proportion of nitrogendioxide (NO,) is only 3-25% as compared withnitric oxide (NO). In urban air, NO, is formed bythe reaction of NO with ozone (0,). In roadtunnels with high NO concentrations, 0, is de-pleted without causing significant conversion ofNO to NO,. An additional problem during con-gestions, and in long road tunnels, is the elevatedproportion of NO,, due to thermal oxidation ofNO. At normal urban atmospheric conditions thisreaction is of minor importance, while the forma-tion rate of NO, is as high as approximately 1ppm/h at an NO concentration of 5 ppm [7]. Sincethe reaction rate is exponentially dependent onthe NO concentration, the NOz formation in-creases fast in road tunnels during congestionswith NO concentrations much higher than 5 ppm.3.3. Human exposure

Traffic exhaust is rapidly diluted by horizontalas well as vertical mixing. Consequently, concen-trations decrease rapidly with distance from theexhaust pipes. This is the reason why the timespent in vehicles is very important for publicexposure to air pollutants. The high exposure ofroad commuters has been demonstrated in studiesfrom Boston [S] as well as from Giiteborg [9]. It isconcluded that the concentration ratios inGoteborg are approximately 1:10:50 betweencommuter trains, commuter cars on roadways,and air in road tunnels. The exposure of profes-sional drivers should be specifically consideredbecause of their long average exposure times.The air pollution problem is particularly seriousin long tunnels because of higher pollution levelsand a longer time of exposure. Reported concen-

trations of hydrocarbons in a 3 km long roadtunnel in Brussels [2] indicate levels about twice ashigh as in the 0.5 km long Tingstad tunnel. This isso in spite of mechanical ventilation and fewervehicles per hour.3.4. Health hazards

With respect to short-time effects, nitrogendioxide is commonly regarded as the most criticalurban air pollutant. In Sweden, the WHO limitfor one-hour exposure (400 pg/m3) is presentlyconsidered as a 98-percentile limit for road tun-nels. The limit to be permitted heavily influencesthe investments in mechanical ventilation. Nitro-gen dioxide affects respiratory organs [lo], andasthmatics and children are particularly suscepti-ble. Synergistic effects with other irritating com-pounds such as aldehydes reduce the acceptablemaximum concentration of nitrogen dioxide. Thepotential connection between the increasing num-ber of allergic persons and exposure to irritatingair pollutants, such as nitrogen dioxide, shouldalso be considered [lo].Exposure in tunnels also contributes to long-term effects among which cancer and othergenotoxic effects are regarded as particularly im-portant. Air pollutants from traffic include manycarcinogenic compounds. Among the hydrocar-bons in Table 1, benzene is a feared humancarcinogen [ 1 ] whereas 1,3-butadiene, ethene andpropene have been put forward as important can-cer risks more recently [ 121.In Sweden, ambient air hygienic threshold vaLues (low-risk levels) are proposed for benzene (1.3pg/m3), 1,3-butadiene (0.05-0.2 pg/m3), ethene(1.2 pg/m) and propene (1.7-17 pg/m) [13].These low-risk levels theoretically result in onecancer case per 100 000 exposed individuals dur-ing life-time inhalation. For toluene, a low-risklevel of 38 pg/m is proposed with reference toeffects on the central nervous system.Human exposure to benzene, butadiene, etheneand propene has recently been estimated to causeapproximately 100 cases of cancer per year inSweden, as calculated from lifetime unit risk fac-tors [12]. In Table 2, the same approach is used tocalculate the cancer risk due to the exposure levels


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434 G. Barrefors / The Science of the Total Environment 189/190 (1996) 431-435Table 2Estimated individual cancer risk from exposure to volatile hydrocarbons, when driving through a 13 km long road tunnel (aspresently planned in Stockholm)

Unit risk factop Unit risk factorb(per pg/m3 x 103 (per pg x 10) Dose (15 min)c6%)Risk of cancer(x 109) Risk of cancer(per yeaP x 106)

Ethene 125 25 63 16 8Fropene 20 4 22 1 11,3-Butadiene 600 120 6 7 4Benzene 16 3 14 2 1a Calculations based on these lifetime unit risk factors [ 121.b The concversion from lifetime unit risk factors @g/m31 is made assuming an average inhalation of 20 m3/day during 70 years.c Inhaled dose during 15 min (15 1 of air/min) in a tunnel assuming the concentrations measured in the Tingstad tunnel (column oneof Table 1).d Based on the assumptions of two tunnel passages a day, 5 days a week, during a year. Considering theoretical and statistica l errors,the figures given are judged to be uncertain by a factor o f three [12].

when driving through a 13 km long road tunnel.The estimations indicate 14 cases of cancer dis-ease per year among one million i&lividualsdriving regularly through the tunnel. Consider-ing the presence of a great number of othergenotoxic compounds in the tunnel air, the can-cer risk of driving through a long road tunnelappears to be unacceptable, especially duringrush hours and congestions.

nant women are pa~ic~~ly vulnerable. Largeinvestments in circumferential roadways in tun-nels cannot be justified with respect to healtheffects.A sustainable mobility requires investmentsfavouring rail rather than road alternatives. Therail tunnel between England and France standsout as a good example, whereas a bridge andtunnel road link across the &esund channelpresents a bad example for a future Europe.

ReferencesThe high concentrations of traffic-emitted airpollutants found in short road tunnels inGijteborg emphasize the hazardous effects to beexpected due to the still higher exposure levelsanticipated in long urban road tunnels plannedin Scandinavia and Europe.

Road tunnels can relieve certain urban areasthat are affected by high levels of vehicle ex-haust. However, if the motorists are taken intoconsideration, not only rural but also urban newroad tunnels normally increase the total popula-tion dose of air pollutants. Moreover, the tun-nels give rise to problems with short-time healtheffects.Private motorists as well as the more heavilyexposed professional drivers have strong reasonsto oppose new road tunnels with respect tohealth hazards. Asthmatics, children and preg-

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[4]

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G. Barrefors and G. Petersson, Volatile hazardous hydro-carbons in a Scandinavian urban road tunnel. Chemo-sphere, 25 (1992) 691-696.P.F. Vande~t~eten, E.L. Wauters and G.E. Verduynm,Tunnel air quality. The carbon balance as an alternativeto evaluate tra ffi c emissions, Staub-Reinhalt, Luft, 51(1991) 83-90.W. Dannecker, B. Schriider and H. Stechmann, Organicand inorganic substances in highway tunnel exhaust air.Sci. Total Environ., 93 (1990) 293-300.W.R. Pierson, A.W. Gertler and R.L. Bradow, Compari-son of the SCAQS tunnel study with other on-roadvehicle emission data. J. Air Waste Manage. Assoc., 40(1990) 1495-1504.I. Savdn (Ed.), Air Pollutants in urban areas, a bibliogra-phy 1985-1991, Swedish Environmental ProtectionAgency, Report 4087, 1992.G. Barrefors and G. Petersson, Assessment of ambientvolatile hydrocarbons from tobacco smoke and fromvehicle emissions. J. Chromatogr., 643 (1993) 71-76.


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G. Barrefors / The Science of the Total Environment 189/19Q (1996) 431-435 435171 0. Lindqvist, E. Ljun~tr~m and R. Svensson, Low ther-

mal oxidation of nitric oxide in polluted air. Atmos,Environ., 16 (1982) 1957-1972.[8] C.-C. Chan, H. Gzkaynak, J.D. Spengler, and L. Sheldon,Driver exposure to volatile organic compounds, CO,ozone, and NO, under differen t driving conditions. Envi-

ron. Sci. Technol., 25 (1991) 964-972.[9] L. Liifgren, K. Persson, A.-M. StromvaI l and G. Petersson,Exposure of commuters to volatile aromatic hydrocarbonsof petrol exhaust. Sci. Total Environ., 108 (1991) 225-233.[IO] M. Berglund, C.-E. BostrGm, G. Bylin, L. Ewetz, L.Gustavsson, P. Mold&s, S. Norberg, G. Pershagen and K.

Victorin, Health risk evaluation o f nitrogen oxides. Scand.J. Work Environ. Health, 19 (suppl. 2) (1993) l-72.[Ii] A. Yardley-Jones, D. Anderson and D.V. Parke, The

toxicity of benzene and its metabolism and molecularpathology in human risk assessment. Br. J. Ind. Med., 48(1991) 437-444.[12] M. Tornqvist and L. Ehrenberg, Cancer risk: Estimationof urban air pollution. Environ. Health Perspect. , 102(suppl. 4) (1994) 173-182.

[13] K. Victorin, Health e ffe cts of urban air pollutants. Guide-line values and conditions in Sweden. Chemosphere, 27(1993) 1691-1706.

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