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Topic 7 Synopsis:

Exposure Assessment to Particulate Organic Compounds

L-J Sally Liu: Contribution of Particulate Organic Compounds to Indoor and Personal Exposures

Wolfgang Rogge: Residential Cooking

Christopher Simpson: Contributions from Outdoor PM Sources to Indoor and Personal PM Exposures

Contribution of Particulate Organic Compounds to Indoor and Personal Exposures

L.-J. Sally Liu, Sc.D., Associate Professor
Department of Environmental & Occupational Health Sciences
University of Washington, Seattle, WA

Organic carbon (OC) is a major constituent of indoor source emissions (Long et al. 2000) and a major component of PM2.5 mass. It may be responsible for some of the observed health effects previously associated with PM exposure. OC contributed an average of 24% of outdoor PM2.5 in U.S. cities, ranging from 17.8% in Detroit to 34.7% in Sacramento (U.S. EPA 2003). Recent receptor modeling for indoor measurements indicated that OC contributed 18% of the indoor PM2.5 in an unoccupied apartment in Baltimore (Hopke et al. 2003) and 65% of indoor PM2.5 in 9 occupied homes in Boston (Long et al. 2000).

The OC contribution to indoor and personal PM2.5 has been difficult to quantify. Many particulate organic compounds (POC) are semi-volatile and exist in equilibrium between the gas and particulate phases. This characteristic provides a challenge of accurate measurement of POC in the presence of artifacts that can occur during sampling. A “positive” sampling artifact (Cui et al., 1997; Turpin et al., 1994; McDow and Huntzicker, 1990; Kim et al., 2001) results from the adsorption of vapor-phase, semi-volatile organic compounds (SVOC) onto the filter that collects particles, or even onto the particles themselves during sampling. A “negative” sampling artifact may result from desorption of vapor-phase SVOC during sampling or subsequent handling (Eatough et al., 1995; John et al., 1988).

For indoor air samples, the net positive sampling artifact was found to be especially significant. Positive artifacts led to overestimation of indoor particulate OC measured with quartz filters in non-denuded Harvard Impactors for PM2.5 by 471% as compared with the average PM2.5 mass concentration measured with Teflon filters in identical samplers and outdoor particulate OC by 153% in the Seattle Exposure and Health Effects Panel Study (Claiborn & Liu, unpublished results). Adsorption saturation of quartz filters for gas phase OC was observed at indoor sites. The average quartz filter saturation level was estimated to be approximately 5.9 mg C/cm2 of quartz filter area. The highest volatility carbon fraction (OC1) is mostly from the adsorption of gaseous OC onto quartz filters during sampling, while the lowest volatility carbon fraction (OC4) fraction is mostly from the particulate phase.

Measurement methods

The tandem-filter method was proposed to account for positive artifact (Fitz, 1990; Turpin et al., 1994). In this method, a single quartz filter is deployed in a sampler that is collocated with a second sampler that contains one upstream Teflon filter and one downstream quartz filter. Another method removes gaseous SVOC in an adsorbent-coated diffusion denuder upstream of the quartz filter. This method makes use of the fact that gases diffuse orders of magnitude faster than particles, allowing the particles to continue through the denuder to be collected on the quartz filter. To solve for the negative artifact, a sorbent or second quartz filter may be placed downstream of the filter. Denuder systems for the removal of gas phase organic compounds are the Brigham Young University Organic Sampling System (BOSS) (Eatough et al., 1993) and the sorbent-coated Integrated Organic Gas and Particle Sampler (IOGAPS) (Gundel and Lane, 1998; 1999). Both samplers are bulky and may not be suitable for deployment as personal samplers or in occupied residences. Pang et al. (2002) developed a promising personal particulate organic and mass sampler (PPOMS) that uses activated carbon-impregnated foam as a combined 2.5-mm size-selective inlet and denuder for assessment of PM2.5 and OC free of the positive artifact.

Alternatively, Fourier transformation infrared (FTIR) spectroscopy can be used to chemically characterize functional group information of outdoor, indoor, and personal PM2.5 samples, especially for those poorly understood organic fraction of PM2.5 (Turpin et al. 2003 submitted). Turpin et al. showed the influence of indoor sources for aliphatic hydrocarbon and amide functional groups by demonstrating enhanced absorbances attributed to these groups in indoor and personal PM2.5 (Teflon) samples. Meat cooking was suggested as a possible source for particulate amides.

Sources of indoor and personal PM2.5

Using a mass-balance model and 16 measured elements, Koutrakis et al. (1992) quantified the contribution of infiltrated outdoor PM and indoor sources including cigarette smoking, wood stoves, and kerosene heaters to indoor PM2.5 in 394 homes in two New York State counties. Yakovleva et al. (1999) used 18 elements analyzed from indoor, outdoor, and personal samples from 178 subjects in Riverside, CA, in a positive matrix factorization (PMF) analysis. They identified major indoor and personal PM2.5 sources to be soil, nonferrous metal operations and motor vehicle exhaust, secondary sulfate, and personal activities. Hopke et al. (2003) utilized both trace elements and OC measurements from indoor and outdoor samples in 3-way PMF and identified nitrate-sulfate, sulfate, OC, and motor vehicle exhaust as major indoor and outdoor sources. Their multilinear engine results indicated that sulfate (46%), personal activities (36%), unknown sources (14%), soil (3%), gypsum (0.7 %), and personal care products (0.4%) are major sources of personal PM2.5 exposure among elderly subjects. Using 3-way PMF, Larson et al. identified vegetative burning (41%), crustal materials (33%), secondary sulfate (19%), mobile vehicle exhaust (7%) as major personal PM2.5 sources.

Recommendations for future work

  1. Validate and cross-compare personal samplers that take into account the OC positive and negative artifacts.
  2. Develop methods to minimize the positive artifacts in existing OC measurements for exposure characterization.
  3. Identify and characterize particulate organic compounds in indoor air and personal exposure and the utilization of these species in source apportionment analysis.

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Residential Cooking

Wolfgang F. Rogge, Ph.D., P.E.
Acting Department Chair and Associate Professor
Department of Civil & Environmental Engineering
Florida International University, Miami, FL

In urbanized areas, restaurant and residential cooking often contributes 30% and more to the atmospheric fine particulate organic particle burden. Whenever no smoking occurs in homes, residential cooking has been suggested to be the major indoor source for respirable particulate matter. In order to fully understand the impact of cooking on indoor air quality and eventually on the overall urban air quality, we first have to have emission factors for all major pollutants.

For that purpose, a specially designed airtight environmental chamber made of stainless steel was designed and built (3.7 m (L), 3.0 m (W), 2.5 m (H)). During the cooking studies, the chamber was operated under controlled airflow conditions, cleaning the incoming air first with an activated carbon filter bed, prefilter, and HEPA-filtering system.

Altogether, more than 140 cooking experiments were conducted and the emission rates determined for: CO, NO, NO2, PM2.5, PM10, TSP. In addition, the PM2.5 samplers were followed by canisters loaded with polyurethane foam (PUF) plugs. The PM2.5 and PUF samples were furthermore subjected to detailed molecular analysis. Cooking was conducted using both electric and natural gas powered ranges and ovens. Many different food items were cooked using pan-frying, stir-frying, sautéing, deep-frying, boiling, and oven cooking methods including baking, roasting, and broiling.

TSP emissions were highest for pan-frying, with up to 11000 mg/kg of food cooked and varied drastically, depending on the food item pan-fried. Using natural gas, the TSP emissions were on average about 15% higher than observed for pan-frying with an cooking range powered by electricity. The next prominent cooking method was oven-broiling steaks, with TSP emissions up to 4300 mg/kg of food cooked. For this cooking method, oven broiling with natural gas as power source caused 10 times higher emissions than what was observed for oven broiling with electricity. In contrast, particulate emission factors for deep-frying did change very little for different food items. Boiling food with water revealed the lowest particulate emissions.

Depending on the cooking method, about 20% to 100% of the TSP was made of PM10, and 30% to 70% of the PM10 consisted of PM2.5. In most cases, an increase in food fat content increased as well particulate emissions.

For most of the cooking experiments conducted, PM2.5 samples and PUF samples were analyzed for individual organic compounds using GC/MS and HPLC. Close to 150 individual organic compounds were quantified, including: n-alkanes, n-alkanoic acids, n-alkenoic acids, n-alkanols, n-alkanals, n-alkan-2-ones, dicarboxylic acids, furans, furanones, amides, steroids, polycyclic aromatic hydrocarbons (PAHs), heterocyclic aromatic amines (HAAs), and others. In general, oven roasting, broiling, and baking resulted in the highest PAH emission factors, ranging on average from about 10 mg/kg of food cooked to about 80 mg/kg of food cooked. In contrast, sautéing and stir-frying generated PAH emissions from about 2 mg/kg to 5 mg/kg. The PM2.5 and PUF samples were as well analyzed for 14 HAAs, of which 8 HAAs could be routinely identified and quantified, including: MeIQx, DiMeIQx, PhIP, AaC, Trp-P-1, Trp-P-2, harman, and norharman. The highest total HAAs emission factor was observed for pan-frying bacon, with about 3 mg/kg.

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Contributions from Outdoor PM Sources to Indoor and Personal PM Exposures

Christopher D. Simpson, Assistant Professor
Department of Environmental & Occupational Health Sciences
University of Washington, Seattle, WA

The US population typically spends a majority of its time indoors, and a major fraction of our PM exposures are encountered in indoor environments. (Klepeis,'01; Klepeis,'96) Nevertheless, PM of outdoor origin (ambient PM) has been shown to infiltrate indoors efficiently and accounts for a significant fraction of the population’s PM exposure. Furthermore, epidemiological evidence links ambient PM exposures with adverse health consequences (Samet,'00; USEPA,'01), and it is ambient PM that is regulated under the NAAQS. Therefore it is important from both a health and a regulatory standpoint to understand the contributions from outdoor PM sources to indoor and personal PM exposures.

Recent studies have highlighted the value of source-specific organic chemical tracers - “molecular markers” – to identify and quantify the contributions of specific sources to ambient PM (Fraser,'03; Khalil,'03; Manchester-Neesvig,'03; Schauer,'00; Schauer,'96; Zheng,'02). Examples of molecular markers for biomass smoke include levoglucosan and methoxyphenols (Fine,'01; '02; Hawthorne,'92). Examples of molecular markers for vehicle exhaust include polycyclic aromatic hydrocarbons, hopanes and steranes (Fraser,'03; Schauer,'99; Schauer,'96), and specific nitro-arenes (e.g. 1-nitropyrene) (Hayakawa,'00). This presentation aims to review the state of the science regarding the use of organic tracers to quantify indoor concentrations of and personal exposures to ambient PM. In particular, I shall focus on PM derived from diesel exhaust and biomass combustion.

Woodsmoke tracers:
A variety of metrics have been used to assess exposures to biomass smoke including measurements of PM mass, CO, OC, potassium (Khalil,'03; Larson,'94; Maykut,'03). Several organic tracers have also been used, including methyl chloride, PAHs, levoglucosan and methoxylated phenolic compounds (methoxyphenols) (Khalil,'03; Larsen,'03; Schauer,'00).

Levoglucosan is an anhydro sugar derived from the pyrolysis of the major wood polymer cellulose. Levoglucosan is one of the the most abundant particle associated organic compounds in woodsmoke (Fine,'01; '02). It is stable in the environment and has been used extensively to estimate woodsmoke levels in ambient PM samples (Katz,'04; Schauer,'00). Levoglucosan is present in other biomass smoke samples including smoke from tobacco, grasses and rice straw (Sakuma,'80; Simoneit,'98; Simpson). However, under conditions in which woodsmoke dominates the biomass smoke contribution to ambient aerosol, levoglucosan can be considered a unique tracer for woodsmoke. In a recent report measurements of levoglucosan in filter samples were used to validate the assignment of the woodsmoke feature in a PMF based source apportionment in indoor, outdoor and personal PM samples.

(Larson,'04) It has been reported that typical cooking conditions do not generate the temperatures required to produce appreciable amounts of levoglucosan. However in our Seattle panel study we observed that for several residences, the 24hr average levoglucosan concentrations were higher indoors than outdoors in a number of samples. Therefore, the possibility remains that indoor sources of levoglucosan exist that would confound its use as a tracer for ambient woodsmoke penetrating indoors.

Methoxyphenols are a class of chemicals derived from the pyrolysis of the wood polymer lignin. This class of chemicals span a range of volatilities from relatively volatile (e.g. guaiacol) to exclusively particle associated (e.g. sinapinaldehyde). These chemicals are relatively abundant in woodsmoke, albeit the most abundant compounds are predominantly in the vapor phase (Hawthorne,'89; Schauer,'01). Smoke from hardwood versus softwood burning can be distinguished by the relative proportions of substituted guaiacols compared to syringols.

Diesel tracers:
A variety of metrics have been used to measure diesel exhaust particles (DEP) in ambient samples, including measurements of ultrafine particles, elemental carbon (EC) PAHs, hopanes and steranes (Manchester-Neesvig,'03; Schauer,'03; Schauer,'99). Indeed the NIOSH method for assessing diesel exposures prescribes measurement of EC. However, these methods are not specific for DEP, and may be confounded by non-diesel sources. Some PAHs (e.g. BghiP) are enriched in DEP relative to other combustion sources, and ratios of BghiP to other PAHs (e.g. BghiP/iP, BghiP/BaA) have been proposed as marker for DE. Hopanes and steranes are present in engine lubricating oil, which comprises a significant component of the particle mass emitted from both gasoline and diesel vehicles. (Manchester-Neesvig,'03; Schauer,'99) EC is enriched in diesel exhaust relative to gasoline exhaust, therefore the abundance of hopanes and steranes relative to EC can be used to differentiate gasoline from diesel emissions. (Fine,'04; Manchester-Neesvig,'03).

Several nitroarenes are enriched in DEP, and some nitroarene isomers appear to be highly specific markers for DEP. (Hayakawa,'00) 1-nitropyrene (1-NP) is one of the most abundant particle-phase PAHs in DEP, at a concentration of ~10-40 ppm . (Bezabeh,'03) Photochemical nitration of pyrene in the atmosphere forms specifically 2 and 4-nitropyrene, and other combustion sources produce minimal amounts of 1-nitropyrene. Therefore, 1-nitropyrene has been proposed as a unique marker for DEP in ambient PM. Although levels of this compound are relatively low in ambient air (examples) (Bamford,'03; Hayakawa,'00) analytical methods based on HPLC with fluorescence of chemiluminescence detection (Hayakawa,'00; Tang,'03), or GC-NICI-MS (Bamford,'03; Bezabeh,'03) have adequate sensitivity to detect 1-NP in low volume ambient samples.

Issues with the use of organic sources tracers in indoor and personal samples:
Given the success in using organic source tracers to apportion PM in outdoor (ambient samples), it is reasonable to apply the molecular marker approach to source apportionment of indoor and personal PM exposures also. However, the applicability of individual markers as suitable tracer compounds indoors should be carefully considered. Some potential concerns that should be addressed when using organic tracers to quantify indoor concentrations of and personal exposures to ambient PM are discussed below.

Tracer fidelity:
Compounds that are unique tracers for outdoor sources may be confounded by contributions from additional indoor sources (e.g. incense burning may confound woodsmoke tracers).

Gas-particle partitioning:
For tracers that are semi-volatile, the particle –vapor equilibrium will shift away from the particle phase as one moves indoors, due to elevated temperatures and the presence indoors of surfaces to adsorb vapor-phase chemicals.

Particle characteristics:
The physical characteristics of the individual particles may be important also. Infiltration efficiencies are highest for the size fraction 0.1-0.4 mm, and decline substantially for particles both smaller and larger than this range. Thus, where outdoor sources produce PM with different size distributions, we may expect to observe differential penetration efficiencies for the different sources of PM. We should also be aware that the concentrations of molecular markers which adsorb to particle surfaces will maximize in a different size fraction than PM mass (Fine,'04).
Sensitivity: Typically indoor and personal samples are collected at low volumetric flowrates (2-10 L/minute). Thus, sampled volume and mass collected will be much lower for indoor/personal samples compared to ambient samples, and the analytical detection limits (expresses as ng/m3) will consequently be higher. These considerations may dictate the use of more sensitive analytical methods or more extensive sample extraction/pre-concentration procedures to optimize sensitivity (Simpson,'04b). We may also have to accept a higher proportion of data close to the detection limits and missing data, and a concomitant increase in error in the analytical method.

Continuous data:
With measurements of PM mass it has recently been demonstrated that continuous data has great advantages for calculating ambient PM infiltration to indoor and personal samples. (Allen,'03) It is also recognized that indoor and personal PM exposures show greater short-term temporal variability that is characteristics of ambient PM. Instrumentation is available that provides continuous data for chemical classes of particle bounds organics (e.g. PAHs). Homeland security concerns are driving rapid technological development of continuous monitoring technology for bioaerosols. As a result, we can look forward to new generation analytical technologies capable of providing continuous data for specific particle bound organic tracers in the near future.

Obtaining accurate measures of personal exposure and, more importantly, absorbed dose, for particulate air pollution is inherently difficult. This is due to the substantial spatial and temporal variation in pollutant levels, coupled with the fact that people constantly move between different microenvironments. Thus, traditional fixed site monitors fail to capture the full variability in exposures experiences by individuals. While active personal monitors are effective in accurately monitoring personal exposures, it is impractical and cost prohibitive to implement active personal monitoring on a large scale. Furthermore, external personal monitors fail to account for substantial differences in ventilation volume, and hence inhaled dose, due to physical exertion. An alternative approach to exposure assessment, which addresses many of the limitations noted above is biomonitoring. Biomonitoring of exposure to particulate air pollution involves measurement of PM-derived chemicals in biological media such as human urine, blood, or hair, and the chemical so monitored is called a biomarker.

A suitable biomarker for specific PM sources should possess the following characteristics:

  1. It should be uniquely derived from the exposure of interest.
  2. It should be relatively abundant, such that ambient exposure levels generate sufficiently high biomarker levels to be measured reproducibly.
  3. The parent marker should be chemically stable in the environment, and the compound (or its metabolites) should be chemically stable in biological samples.
  4. The excretion kinetics in the media of interest (e.g. urine, blood etc) should be suited to the exposures and/or health endpoints of interest.

Biomarker for woodsmoke exposures:
Three classess of chemicals have been proposed as biomarkers for woodsmoke, PAHs, levoglucosan and methoxyphenols have all been proposed as biomarkers for woodsmoke exposure (Dills,'01; Dorland,'86; Feunekes,'97; Rothman,'93).
Feunekes et al. measured 1-hydroxypyrene in urine from firefighters. (Feunekes,'97) They reported a positive association between urinary 1-hydroxypyrene and smoke exposure. In contrast, Rothman et al reported no association between PAH-DNA adducts in peripheral blood from wildland firefighters, and smoke exposures. (Rothman,'93) PAHs are by no means specific to woodsmoke; they are a component of incomplete combustion and are present in a variety of PM sources including vehicle exhaust, gas and coal combustion and cooking fumes (Rogge,'91; '93; Schauer,'99). Furthermore, for non-occupationally exposed non-smokers, the major portion of PAH dose is taken up through the diet (Chuang,'99; Vyskocil,'00). Therefore, PAH biomarkers in urine or blood are only likely to be associated with PM exposures when woodsmoke exposures are very high, such as occupational exposures.

There is one report describing measurement of levoglucosan in human urine (Dorland,'86). Dorland et al used an approach combining TLC and GC/MS, and reported 0 to 0.8 mg levoglucosan per mL of urine. This finding should be replicated using modern techniques combining high resolution chromatography and mass spectrometry as the analytical methods used in the original study have limited specificity. The upper range of the urinary levoglucosan levels reported by Dorland et al are higher than would be achieved from inhalational exposure to woodsmoke at ambient levels, and the possibility exists of a substantial dietary contribution of levoglucosan from caramelized sugars.

Our group has described the use of methoxyphenols as potential biomarkers of woodsmoke exposure (Dills,'01). We have developed sensitive and specific methods based on GC/MS analysis for determination of approximately 12 methoxyphenols in human urine. A suite of isotopically labeled methoxyphenools are used to monitor analyte recovery in every sample. Multiple methoxyphenols are present in the urine of individuals with no known elevated exposure to woodsmoke, and a substantial increase in urinary methoxyphenol excretion was reported subsequent to inhalation of woodsmoke from a campfire (Dills,'01). It was also noted that ingestion of food items containing woodsmoke flavoring (e.g. smoked salmon) caused a substantial increase in urinary methoxyphenol excretion (Dills,'01). The utility of methoxyphenols as a biomarker for woodsmoke xposure at ambient levels was evaluated in a panel study in Seattle WA. Mutiple methoxyphenols were detected in all urine samples, and a dynamic range up to 1000-fold between lowest and highest reported concentrations was observed.

Biomarkers for diesel exposures:
Several biomarkers for diesel exhaust have been proposed, including PAH metabolites and various nitro-PAH metabolites in urine samples (short term exposure markers) and as adducts to DNA or blood proteins (longer term exposure biomarkers. (Kuusimaki,'03; Scheepers,'02; Seidel,'02; van Bekkum,'97) PAH metabolite levels in urine samples have shown an association with DE exposure in some occupational settings, (Kuusimaki,'03) however, as noted above, PAHs are not source specific tracers. Thus, for exposure to ambient levels of DE, and where substantial not DE sources of PAHs are present, urinary PAH metabolites are not likely to be useful biomarkers for DE exposure. In contrast, several nitro-arenes are unique to (or at least greatly enriched in) DE, and their urinary metabolites hold promise as sensitive and specific biomarkers of DE exposure. Urinary metabolites of 1-nitropyrene and 3-nitrobenzanthrone have been reported in subjects with known occupational exposure to DE. (Seidel,'02) Unfortunately in these studies concurrent personal PM samples were not collected so dose-response information for the nitro-PAH metabolites in human urine is not yet available. Future work with these biomarkers should involve establishing dose-response and timecourse of urinary excretion, adapting analytical methodology to permit the sensitive and specific determination of these markers in urine from non-occupationally exposed individuals, and established baseline urinary levels and variance in individual exposed to ambient DE.

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The Organic Speciation International Worskhop is sponsored by the Western Regional Air Partnership/Western Governors Association. APACE is seeking support from the US Dept. of Energy, US EPA Office of Air Quality Planning and Standards, and the National Science Foundation.