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Particulate Air Pollution from Bushfires:Human Exposure and Possible HealthEffectsSathrugnan Karthikeyan a , Rajasekhar Balasubramanian a b &Kostetski Iouri ca Department of Chemical and Biomolecular Engineering , NationalUniversity of Singapore , Singaporeb Division of Environmental Science and Engineering , NationalUniversity of Singapore , Singaporec Division of Bioengineering, Faculty of Engineering , NationalUniversity of Singapore , SingaporePublished online: 05 Dec 2006.

To cite this article: Sathrugnan Karthikeyan , Rajasekhar Balasubramanian & Kostetski Iouri(2006) Particulate Air Pollution from Bushfires: Human Exposure and Possible Health Effects,Journal of Toxicology and Environmental Health, Part A: Current Issues, 69:21, 1895-1908, DOI:10.1080/15287390600751264

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Journal of Toxicology and Environmental Health, Part A, 69:1895–1908, 2006Copyright© Taylor & Francis Group, LLCISSN: 1528–7394 print / 1087–2620 onlineDOI: 10.1080/15287390600751264

PARTICULATE AIR POLLUTION FROM BUSHFIRES: HUMAN EXPOSURE AND POSSIBLE HEALTH EFFECTS

Sathrugnan Karthikeyan1, Rajasekhar Balasubramanian1,2, Kostetski Iouri3

1Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore2Division of Environmental Science and Engineering, National University of Singapore, Singapore3Division of Bioengineering, Faculty of Engineering, National University of Singapore, Singapore

Toxicological studies have implicated trace metals adsorbed onto airborne particles as possiblecontributors to respiratory and/or cardiovascular inflammation. In particular, the water-solublemetal content is considered to be a harmful component of airborne particulate matter. In thiswork, the trace metal characteristics of airborne particulate matter, PM2.5, collected in Sin-gapore from February to March 2005 were investigated with specific reference to their bio-availability. PM2.5 mass concentrations varied between 20.9 mg/m3 and 46.3 mg/m3 with anaverage mass of 32.8 mg/m3. During the sampling period, there were several bushfires in Sin-gapore that contributed to sporadic increases in the particulate air pollution, accompanied byan acrid smell and asthma-related allergies. The aerosol samples were subjected to analysis oftrace elements for determining their total concentrations as well as their water soluble frac-tions. Our results showed an increase in concentration of several water-soluble trace metalsduring bushfires compared to their urban background levels in Singapore. In order to measurethe human exposure to particulate air pollution, the daily respiratory uptake (DRU) of severaltrace metals was calculated and compared between haze and nonhaze periods. The DRU val-ues were significantly higher for several metals, including Zn, Cu, and Fe, during bushfires. Elec-tron paramagnetic resonance (EPR) measurements showed that the particulate samplescollected during bush fires generate more toxic hydroxyl radicals (OH·) than those in the back-ground air, due to the presence of more soluble iron ions.

Exposure to ambient particulate matter (PM) was reported to be associatedwith the incidence of acute mortality and morbidity due to cardiovascular orpulmonary complications (Dockery et al., 1993; Dominici et al., 2005; Popeet al., 1995). PM was also associated with increased rates of lung cancer

Accepted 7 December 2005.A part of this study was funded by the NUS ARF through grant RP-279-000-142-112. We are grateful

to the Southeast Asia Regional Committee for START and National Central University, Taiwan, for financialassistance. The authors thank Prof. Nikolai Kocherginsky for allowing us to use the EPR facility at the Divi-sion of Bioengineering, NUS, and Ellis See and Umid Joshi for assistance with the field sampling.

Address correspondence to Rajasekhar Balasubramanian, Division of Environmental Science andEngineering, 2 Engineering Drive 1, National University of Singapore, 117576 Singapore. E-mail: eserbala@nus.edu.sg

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1896 S. KARTHIKEYAN ET AL.

(Cohen & Pope, 1995; Krewski et al., 2005). A recent epidemiological studyreported that every 10-mg/m3 increase in fine particles increased the risk fordeath from lung cancer by 8% (Pope et al., 2002). These problems are likely tohave a higher occurrence in populations living in large metropolitan areas withmultiple sources (Jerrett & Finkelstein, 2005). The concern over the healtheffects of PM has led to intensive research on atmospheric fine particles, PM2.5(airborne particulate matter with a mass mean aerodynamic diameter of lessthan or equal to 2.5 mm), in several countries, including Singapore. UrbanPM2.5 is a complex mixture of primary emissions from industry, transportation,power generation, and natural sources and of secondary material formed bygas-to-particle conversion mechanisms. To gain a better understanding of therelative contribution of these sources to the atmospheric loading of PM andprovide realistic health risk assessment due to its exposure, the monitoringdata on PM2.5 and its chemical composition are needed.

PM2.5 contains a complex mixture of aggregates of organic and inorganiccompounds such as carbonaceous material, polycyclic aromatic hydrocarbons,salts, metals, and endotoxins. Some of the chemical components in urban aero-sols, such as metals and, especially, their soluble fraction, have attracted particu-lar attention because of their potential health effects (Adamson et al., 1999,2000; Dreher et al., 1997; Dye et al., 2001; Ghio & Delvin, 2001). It has beenspeculated that the bioavailability of metals from PM2.5 may have a more directlink to adverse health effects than total metal concentrations (Ghio et al., 1992,1996). This possibility led to the completion of several recent studies thatassessed water-extractable metal concentrations from ambient samples in areaswith elevated PM2.5 concentrations. Prieditis and Adamson (2002) evaluated thecomparative pulmonary toxicity of various soluble metals found in urban partic-ulates, and reported that Zn and Cu are most likely to produce lung injury andinflammation as compared to metals such as Ni, Pb, and V at the same concen-tration levels. It was reported that trace metals, particularly iron, released fromairborne fine particles or present on their surfaces play a role in the generation ofreactive oxygen species (ROS), and this is also true for coal fly ash particles(Donaldson et al., 1997; Dornisch et al., 1999; Dellinger et al., 2001; Dreher etal., 1997; Goldsmith et al., 1998; Van Maanen et al., 1999). The degree ofmetal solubility may therefore influence the toxicity of a given PM sample.

Atmospheric fine particles in Singapore are generated not only from emis-sions of common combustion sources such as power plants and motor vehicletraffic, but also from wildfires in the region (Balasubramanian et al., 2003; Bala-subramanian & Qian, 2004). Forest and peat fire emissions from Indonesiaelevated the PM2.5 concentrations in Singapore several times over the lastdecade, leading to reduced atmospheric visibility (haze) and respiratory ailments(Orlic et al., 1999; Balasubramanian et al., 1999, 2003; Emmanuel, 2000). Thechemical composition of PM2.5 particles was measured during both haze andnonhaze periods to determine the relative contributions of various sources to theparticulate air pollution in Singapore. However, the soluble fraction of trace met-als in PM2.5, which poses a potentially high health risk, remains to be investigated.

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In this study, the total concentration and the water-soluble metal fractionwas compared for a selection of trace metal species in urban PM2.5 samplescollected in Singapore over a period of 30 days (February–March 2005), withthe aim of updating aerosol science knowledge necessary for health risk assess-ment. During the course of this research, Singapore experienced a recordnumber of bushfires sparked by the scorching dry spell, leading to hazy skyconditions and accompanied by an acrid smell. An in-depth characterizationof this smoke haze episode was undertaken with specific reference to the bio-availability of Al, Co, Cu, Fe, Mn, Pb, Ni, Zn, Cd, Ni, Ti, V, and As in hazeaerosols. The daily respiratory uptake of heavy metals was calculated andcompared between haze and nonhaze periods to assess human exposure towater-soluble metals through inhalation. In addition, the role of soluble ironions was investigated in the generation of hydroxyl radicals (OH·), in aqueousbuffered solutions, in the presence of hydrogen peroxide, using electron para-magnetic resonance (EPR).

MATERIALS AND METHODS

Sampling Site LocationThe sampling site is located at the rooftop of one of the tallest buildings

(67 m above sea level) within the National University of Singapore campus.This building houses classrooms and research laboratories, which are mechan-ically ventilated by drawing ambient air through a network of air handlingunits. Thus, the indoor air quality of the building is closely related to the ambi-ent air quality.

Singapore is located at the southern tip of the Malayan Peninsula, betweenlatitudes 1°09¢ N and 1°29¢ N and longitudes 103°36¢ E and 104°25¢ E, andmeasures 42 km from east to west and 23 km north to south. The site isapproximately 800–1000 m away from the open sea. The sampling site isinfluenced by emissions from chemical industries, major power plants, andoil refineries situated in a group of small islands on the west coast of theSingapore Island and also by urban vehicular traffic.

Sample CollectionPM2.5 samples were collected using a high-volume sampler (Hi-Q Envi-

ronmental Products; model PM2.5-1000), equipped with an inlet with aPM2.5 size-cut, on quartz filters, over a period from February 20 throughMarch 20, 2005. During the sampling period, a number of bushfire eventsoccurred in Singapore. The filters were preequilibrated in a dry box with astabilized temperature (22–23°C) and relative humidity (45–50%) for at least24 h before the actual weighing. Mass concentrations of PM2.5 were calcu-lated by subtracting the precollection weight from the postcollection weightand dividing by the volume of air that was pulled through the filter over thesampling period.

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Metal AnalysisTo determine water-soluble metal, a portion of filters (one circle 2.5 cm

diameter) was ultrasonicated at room temperature for 1 h in 15 ml ultrapure18.2 MW water, after which 10 ml of extract was removed and acidified to 2%HNO3 to minimize metal adsorption. For total metal, another portion of filters(one circle 2.5 cm diameter) was subjected to microwave digestion using nitricacid (HNO3)–hydrogen peroxide (H2O2)–hydrofluoric acid (HF) acid mixture(4 ml:2 ml:0.5 ml). The details of microwave digestion are described else-where (Swami et al., 2001).

Quantification of Al, As, Cd, Cu, Cr, Fe, Mn, Ni, Pb, Ti, V, and Zn in thetwo sets of extracts was performed using a Perkin Elmer Elan 6100 inductivelycoupled plasma–mass spectrometer (ICP-MS). Where possible, two or moreisotopes were monitored as a cross-check against potential interference byother species of the same, or closely overlapping, m/z value. An 11-point cali-bration, diluted from multielement standards, was used for each ICP-MS run.Metal concentrations were expressed as nanograms per cubic meter.

Measurement of Hydroxyl Radical (OH·) Produced From Particulate Samples Using EPRThe experimental protocol employed in this study is similar to the one

used by Valavanidis et al. (2000). A portion of the particulate filter was cut intosmall pieces and placed in a conical flask with 40 ml of phosphate buffer(0.01 M, pH 7.4). Extraction was carried out under sonication for 30 min. Theextracts were centrifuged at 2500 rpm for 5 min and particles were suspendedand concentrated into a 5-ml volume. EPR measurements were performedwith solutions containing 50 mM H2O2 and 0.05 M of the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) at 9.76 GHz (X-band) hyperfrequencyusing a Elexsys series E500 CW-EPR X-band (9–10 GHz) Bruker spectrometer(Bruker BioSpin Gmbh, Rheinstetten/Karlsruhe, Germany), installed at theBiophysics Laboratory, Division of Bioengineering, National University ofSingapore (NUS). The EPR spectra were recorded at room temperature. Priorto use, DMPO was purified with activated charcoal. The experimental param-eters were as follows: 100 kHz modulation frequency, modulation of ampli-tude of 1 to 2 G, 20 mW microwave power, receiver gain of 60, and timeconstant of 0.082 s. The resonance relation for EPR is given by:

where n is the hyperfrequency (MHz) and H the magnetic field (gauss).

Quality AssuranceThe precision and accuracy of the extraction procedure were evaluated

using the NIST airborne particulate matter standard reference material SRM

g v H= 0 714484. /

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PARTICULATE AIR POLLUTION EXPOSURE FROM BUSHFIRES 1899

1648. SRM samples in triplicate, with masses between 5 and 10 mg, wereaccurately weighed and analyzed as already described for total and water-soluble fraction of trace elements. The recoveries of total metals from NIST1648 ranged between 90 and 110% for all 12 elements analyzed. For thewater-soluble fraction, the recovery varied from <25% on average for Al, Fe,and Ti to >60% for As, Cd, Pb, Cu, Co, Ni, Mn, Zn, and V, respectively. Theseresults are in good agreement with Desboeufs et al. (2005).

RESULTS

PM2.5 ConcentrationsFigure 1 shows PM2.5 mass concentration during the study period, along

with the Pollutant Standards Index (PSI) data. The PSI. as developed by the U.S.Environmental Protection Agency (U.S. EPA), is used in Singapore for thereporting of daily air pollutant concentrations. The PSI converts the daily airpollutant concentrations measured at a network of air quality monitoring sta-tions, maintained by the National Environmental Agency, to a simple numberon a scale of 0 to 500. Intervals on the PSI scale are related to the potentialadverse health effects of the daily measured concentrations of the five major airpollutants, SO2, PM10, NO2, O3, and CO; most of the time, the PSI reading forthe day is mainly determined by the concentration of PM10. The PSI intervalsare indicative of the prevailing air quality as follows: 0–50, good; 51–100, mod-erate; 101–200, unhealthy; 201–300, very unhealthy; and >300, hazardous.

FIGURE 1. Temporal variation of PM2.5 concentration.

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1900 S. KARTHIKEYAN ET AL.

As can be seen from Figure 1, both the PM2.5 mass concentrations and thePSI values track each other very closely. While the PM2.5 mass concentrationvaried from 20.9 mg/m3 to 46.3 mg/m3, with an average of 32.8 mg/m3, the PSIranged between 30 and 58. During the study period, there were a number ofbushfires in Singapore, caused by an extended dry weather condition, whichcontributed to a sporadic increase in the atmospheric loading of the PM onseveral occasions. In fact, there were a total of 509 bushfires in the first 55days of the year, exceeding the 500 recorded for the whole of the previousyear (The Straits Times, February 25, 2005). As a result of particulate emissionsfrom the local bushfires, the PSI hit the moderate range for a third day in row,but was otherwise in the good range, typically between 25 and 40.

Total Trace Elements ConcentrationThe total metal concentrations of PM2.5 (mean ± standard deviation) deter-

mined in this study are presented in Table 1. These concentrations are withinthe range of those previously reported for Singapore (Balasubramanian &Qian, 2004), as well as for the whole of Asia (Fang et al., 2005). Overall, themeasured trace metal concentrations constituted about 3–5% of the PM2.5mass concentration. Al, Fe, and Ti together accounted for about 90% of thetotal metal concentrations measured in this study. The concentrations of anumber of trace elements, particularly Cu, Ti, Fe, Zn, and Al, increased signifi-cantly by a factor of 6.4, 2.7. 2.2, 1.5, and 1.4, respectively, during the smokehaze episodes caused by the local bushfires, compared to their backgroundlevels. All of these elements were reported as being generated during biomasscombustion, either via fine-mode condensation and coagulation, or via release

TABLE 1. Summary of Trace Metal Concentrations in Urban Fine Particles in Singapore

Total (mean ± SD) Water-soluble (mean ± SD) DRUa (mean, ng/day)

Elements Background During bushfires Background During bushfires BackgroundDuring

bushfires

Al (ng m-3) 519 ± 91 719 ± 219 8.5 ± 4.5 23.6 ± 10.9 149 473Co (ng m-3) 0.3 ± 0.1 0.5 ± 0.2 0.17 ± 0.07 0.35 ± 0.19 3 7Cu (ng m-3) 8.3 ± 2.9 53 ± 39 3.9 ± 2.3 10.4 ± 5.1 77 207Fe (ng m-3) 534 ± 71 1191 ± 507 7.5 ± 3.2 20.8 ± 12.5 149 416Mn (ng m-3) 9.7 ± 2.5 22.7 ± 6.6 1.6 ± 0.6 5.2 ± 1.3 32 105Pb (ng m-3) 11.3 ± 5.6 18.1 ± 7.8 0.9 ± 0.5 0.83 ± 0.20 17 17Zn (ng m-3) 51.3 ± 14.8 79 ± 53 40.0 ± 9.3 74.4 ± 44.1 799 1489Cd (ng m-3) 0.11 ± 0.05 0.4 ± 0.3 0.10 ± 0.02 0.20 ± 0.09 2 4Ni (ng m-3) 4.0 ± 1.2 12.2 ± 11.2 2.3 ± 1.5 3.1 ± 1.6 47 61Ti (ng m-3) 57 ± 43 153 ± 134 1.8 ± 0.9 1.7 ± 0.8 33 36V (ng m-3) 7.8 ± 4.5 20.4 ± 12.1 3.2 ± 1.3 12.7 ± 7.0 63 253As (ng m-3) 1.1 ± 0.3 2.7 ± 2.5 0.52 ± 0.19 0.90 ± 0.39 10 18

a Calculated based on water-soluble fraction.

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PARTICULATE AIR POLLUTION EXPOSURE FROM BUSHFIRES 1901

of incompletely combusted plant tissues and ash or suspension of soil particles(Gaudichet et al., 1995).

Water-Soluble Fraction of Trace MetalsData pertaining to water-soluble trace metals are also included in Table 1.

The mean water-soluble fraction of the total metal concentrations is shown inFigure 2 for 10 trace metals. The solubility of the individual metals rangedfrom as low as <6% for Ti, Fe, and Al to as much as 92% Zn. A substantial pro-portion (40–70%) of Ni, As, V, Cd, and Co was also water-soluble. This obser-vation is in agreement with other studies (Heal et al., 2005; Gilmour et al.,1996). However, the water-soluble proportion of Pb was found to be low (6%)in this study when compared to that reported (30–40%) in previous studies(Heal et al., 2005; Janssen et al., 1997). This difference in the water-solublefraction of Pb could be due to the difference in the nature of Pb speciespresent in airborne particles of different origin, and their water extractability.Overall, the water-soluble fraction contributed 0.33% (0.15–0.47) of the PM2.5mass while the total trace metal accounted for 4.1% (3.1–6.7) of the mass. Themean water-soluble concentrations of metals varied from 0.16 ng/m3 (Cd) to62.35 ng/m3 (Zn) during the study period.

Respiratory Uptake of Heavy MetalsIn order to estimate human exposure to particulate air pollution especially

during the bushfire events and to assess potential adverse health effects, dailyrespiratory uptake (DRU) with respect to airborne metal concentrations was

FIGURE 2. Mean % solubility of trace elements from airborne PM2.5.

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Fe Pb Mn Cu Ni As V Cd Co Zn

Trace metals

slatem ecart fo ytililbulos reta

w % nae

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1902 S. KARTHIKEYAN ET AL.

calculated from the equation DRUi = Ci AR, where Ci is the concentration ofinhaled metals, A is the absorption rate, and R is the daily inhalation rate. Theterm Ci A, represents the bioavailable metal fraction. Inhalation rates are either15 or 20 m3/day, depending on the type of environment where exposure toparticulate air pollution occurs; the former inhalation rate is used for peopleremaining in indoor environments, while the latter is applicable to outdoorenvironments. The respiratory absorption rate for particulate air pollutantsusually varies from 50 to 100% depending on the particle size. In this study,the daily respiratory uptakes of 12 trace metals were calculated by multiplyingthe water-soluble fraction of the total metal concentrations by an inhalationrate of 20 m3/day (Vousta & Samara, 2002). The results are presented in Table1 for background air in Singapore, as well as for the biomass combustionevents caused by the local bushfires. The lowest respiratory uptakes werefound for Cd and Co, while the highest was for Zn. Variations in the respira-tory uptakes of metals may be attributed to differences in their total concentra-tions and their bioavailability, as shown in Figure 3. The DRUs were higher,particularly for Zn, Cu, V, and Fe, during the bushfire events compared to thecorresponding values obtained for the background air.

DISCUSSION

During the course of this study, smoke haze episodes caused by bushfirestook place at least on 4 days. On all 4 days, the PM2.5 mass concentrationsincreased significantly and exceeded the mean concentration recorded duringthe sampling period (32.8 mg/m3) as well as the annual mean concentrationsmeasured in the previous years (22.5 mg/m3; Balasubramanian et al., 2003), as

FIGURE 3. Difference in trace element composition between total and water-soluble fraction (sum of theconcentrations of trace elements = 1777 ng/m3; total concentration of water-soluble elements = 119 ng/m3).

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PARTICULATE AIR POLLUTION EXPOSURE FROM BUSHFIRES 1903

can be seen in Figure 1. This increase in the particulate air pollution is of con-cern since the particles smaller than 2.5 mm in diameter easily bypass the nor-mal body defense mechanism and penetrate deeply into the alveoli of thelungs. It was reported that about 8000 people sought treatment at the govern-ment polyclinics in Singapore for upper-repiratory-tract infections, up from aweekly average of about 6000, when the air quality deteriorated due to emis-sions from the local bush fires. In addition, asthma cases also went up from anaverage of about 500 a week to close to 700 in February (The Straits Times,February 25, 2005).

Similar observations occurred in Singapore during the 1997–1998 smokehaze and also in 2002 in terms of sporadic increases in particulate air pollutionaccompanied by a reduction in atmospheric visibility when emissions frombiomass and peat fires in Sumatra were transported over the long range to sev-eral parts of Southeast Asia (Orlic et al., 1999; Balasubramanian et al., 1999,2003). In fact, the PSI reached the unhealthy range (PSI > 100) on 12 daysduring the 1997–1998 smoke haze episode. The highest PSI recorded was138. Findings from the health impact of the haze showed that there was a 30%increase in outpatient attendances for haze-related conditions (Emmanuel,2000). The increased atmospheric loading of biomass combustion-derivedparticles was associated with an increase of 12% in upper-respiratory-tractillnesses, 19% asthma, and 26% rhinitis (Emmanuel, 2000). Johnston et al.(2002) reported a significant increase in asthma cases in Brisbane, Australia,with an increase of 10 mg m-3 in PM10 mass concentrations, based on an epi-demiological study of exposure to bushfires. Thus, the observations made inthis study on the relationship between the increased particulate air pollutiondue to bushfires and the acute adverse health effects are consistent with thefindings from previous studies (Krewski et al., 2005; Dominici et al., 2005).

Based on the data given in Table 1, it can be seen that a significantincrease in the concentration of Zn, Fe, and Cu became evident during thebushfires episodes. These three metals also accounted for a significant fractionof the water-soluble elements (see Figure 3), and thus resulted in higher DRUsin the same order of abundance. It is important to consider the differences intrace metal solubility from a bioavailability perspective because the predomi-nant exposure pathway for airborne particles to humans is through the air/lungfluid interface (Vousta & Samara, 2002). Since typical lung fluid has a nearneutral pH (Vousta & Samara, 2002), the dissolution experiments carried outin this study suggest that the trace metals, particularly Fe, Cu, and Zn, passingacross the air/lung interface in water-soluble forms are of concern because ofthe following reasons. Toxicological studies showed that inflammation reac-tions in the lung and subsequent tissue damage are associated with biologicallyaccessible metal ions, particularly iron and copper, and the generation of oxi-dants (Ghio et al., 1992; Goldsmith et al., 1998). Valavanidis et al. (2000)reported that the oxidant-generating activity of respirable particles is associ-ated with the amount of soluble iron ion present. Among the reactive oxygenspecies (ROS) generated by PM, the OH· radical is of greatest concern as it is a

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highly reactive electrophilic species known for its ability to attack DNA (Shiet al., 2003a).

Experiments explored the formation of hydroxyl radicals (OH·) in thefreshly prepared water extracts (aerosol suspensions) of the PM2.5 samples col-lected in this study containing the trace metals in the presence of H2O2 basedon the EPR measurements of DMPO–OH adduct for both background aero-sols and those influenced by bushfires. EPR measurement using the spin trapDMPO is an established system to directly determine the formation of OH·

(Lloyd et al., 1997; Shi et al., 2003b). Figure 4 shows representative EPR spec-tra of the aqueous extracts (1a and 1b) and DMPO–OH signals (1:2:2:1 quar-tet) (2a and 2b). This figure clearly indicates that, in the presence of H2O2 andthe spin trap DMPO, hydroxyl radicals (OH·) were generated. The formationof OH· was not observed when blank filter preparations were tested (notshown in the figure). In the absence of H2O2 in the EPR experiments, onlyminimal amounts of OH·were detected. It is well known that OH· productionis the result of ferrous ion-catalyzed degradation of H2O2 via the Fentonreaction:

Hydrogen peroxide was shown to be present endogeneously in living cells(Shi et al., 2003a).

Two interesting observations were found in the EPR spectra: (1) Both 1aand 1b exhibited peaks at g = 2.3 (around 3000 G), which can be ascribed tothe presence of ferrous and ferric iron (Fe2+ and Fe3+) in the form of oxyhy-droxides (Olivie-Lauquet et al., 2000). Spectrum 1a has a broader peak com-pared to 1b, implying that the bushfire-affected samples contained higherconcentrations of Fe. In addition to this peak, the spectrum 1a (affected bybushfires) showed another resonance peak at g = 4.3 (around 1500 G), whichis characteristic of Fe3+ bound to organic groups (organic matter) (Fe3+–OM)(Olivie-Lauquet et al., 2000). The EPR data are consistent with ICP-MS data inthat the bushfire-affected aerosol samples contained higher concentration ofFe compared to the background aerosols, and amount of OH· producedincreased in 2a compared to 2b by a factor of 1.8, as evident from the areasunder the peaks. It was hypothesized that the enhancement in the concentra-tion of OH· in sample 2a, affected by bushfires, is due to a higher concentra-tion of both Fe2+ and Fe3+. Fe3+ can be reduced to Fe2+ in solutions by (Scheff & Valiozis, 1995):

In fact, soluble ferric ions were found to be present in urban airborne par-ticles (Valavanidis et al., 2000). It is quite possible that other redox-active met-als such as Cu might also participate in the generation of OH· radicals. Further

Fe H O H Fe H O OH22 2

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1905

FIG

URE

4.

Repr

esen

tativ

e EP

R sp

ectr

a of

aer

osol

sus

pens

ions

(1a,

bus

hfire

s; 1

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e D

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add

uct (

2a, b

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ires;

2b,

bac

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und)

.

-200

0

-150

0

-100

0

-5000

500

1000

1500

500

1000

150

020

0025

0030

0035

00

4000

450

050

0055

00

6000

650

Fie

ld, [

G]

Intensity, [AU]

-12

00

-10

00

-80

0

-60

0

-40

0

-20

00

200

400

600

800

1000

1200

341

334

2334

33

344

334

533

46

334

7334

83

Fie

ld, [

G]

Intensity, [AU]-1

20

0

-10

00

-800

-600

-400

-200

0

200

400

600

800

100

0

120

0

3413

3423

343

334

43

34

53

34

63

347

334

83

Fie

ld, [

G]

Intensity, [AU]g

=2

.3

g=

4.3

F

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M

Fe2

+

Fe2

+&

Fe3

+ o

xy

-hy

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xid

es 1a

2a2b

-200

0

-150

0

-100

0

-5000

500

1000

1500

2000

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

650

Fie

ld, [

G]

Intensity, [AU]

1b

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1906 S. KARTHIKEYAN ET AL.

research is warranted to gain a better understanding of the formation of oxi-dants such as OH· upon inhalation of fine particles, in view of their ability toinduce damage to DNA and thus induce deleterious health effects. In addi-tion, the synergistic effects of multiple metal ions should also be investigated.

In conclusion, interest in the adverse health effects due to inhaled particleshas increased during recent decades, even though the mechanisms underlyinghealth effects caused by such particles are not fully understood. In this study,the role of bioavailable trace metals that could be of importance for under-standing the mechanisms through which the increased loading of atmosphericfine particles from bushfires induces health effects was investigated. Based onthe results obtained, it is postulated that the deleterious health effects associ-ated with the increased particulate air pollution due to biomass combustionmay be partially ascribed to radicals associated with redox-active metals suchas Fe. With further research on the properties of urban PM2.5 that are responsi-ble for toxicity, more effective air-quality standards and emission control strat-egies could be developed and implemented to reduce the concentration ofthe toxic particles.

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