WHO Guidelines for Indoor Air Quality: Selected pollutants.

Volatile Organic Compounds Sources, Measurements, Emissions, and the Impact on Indoor Air Quality

Ambient particulate matter (PM) is a complex mixture of inorganic and organic compounds. The U.S. Environmental Protection Agency (EPA) regulates PM as a criteria pollutant and promulgates National Ambient Air Quality Standards for it. The PM indicator is based on mass concentration, unspecified as to chemical composition, for specific size fractions. The numerical standards are based on epidemiologic evidence of associations between the various size-related particle mass concentrations as indicators and excess mortality and cardiorespiratory health effects as endpoints. The U.S. National Research Council has stated that more research is needed to differentiate the apparent health effects associated with different particle chemical constituents. Sulfate and nitrate constitute a significant portion of the particle mass in the atmosphere, but are accompanied by similar amounts of carbonaceous material, along with low concentrations of various species, including bioactive organic compounds and redox cycling metals. Extensive animal and human toxicology data show no significant effects for particles consisting only of sulfate and nitrate compounds at levels in excess of ambient air concentrations. A few epidemiologic studies, including both short-term time-series studies and long-term cohort studies, have included the sulfate content of PM as a specific variable in health effect analyses. There are much less data for nitrate. The results from the epidemiologic studies with PM sulfate are inconsistent. A detailed analysis of the time-series epidemiological studies shows that PM sulfate has a weaker "risk factor" than PM2.5 for health effects. Since sulfate is correlated with PM2.5, this result is inconsistent with sulfate having a strong health influence. However, there are many limitations with these types of studies that warrant caution for any comparison between a chemical component and mass concentration. In total, the epidemiologic and toxicologic evidence provide little or no support for a causal association of PM sulfate and health risk at ambient concentrations. For nitrate-containing PM, virtually no epidemiological data exist. Limited toxicological evidence does not support a causal association between particulate nitrate compounds and excess health risks. There are some possible indirect processes through which sulfate and nitrate in PM may affect health-related endpoints, including interactions with certain metal species and a linkage with production of secondary organic matter. There is insufficient evidence to include or exclude these processes as being potentially important to PM-associated health risk.

Evidence of Health Impacts of Sulfate-and Nitrate-Containing Particles in Ambient Air

Concentrations of volatile organic compounds (VOCs) measured indoors may exceed their odor thresholds, but are usually far below TLV estimates. Even applying additivity to eye and airway irritation effects, it is difficult to rationalize increased sick building syndrome (SBS) symptoms by exposure to generally chemically inert VOCs in the indoor environment. Several studies suggest that chemical reactions in indoor air are linked with SBS symptoms and the examination of these reactions may be necessary in order to understand the role of VOCs as causative agents of SBS symptoms. The usual evaluation of odor annoyance of VOCs based on odor thresholds should be modified, taking into account the large vari- ation of individual human odor thresholds for single substances, and specific additivity phenomena even at subthreshold levels of VOCs. The conclusion of this review is that chemical reactions between oxidizable VOCs and oxidants, such as ozone and possibly nitro- gen oxides, can form irritants which may be responsible for the reported symptoms. Compounds adsorbed to particles may also contribute to SBS symptoms. The individual effects of indoor pol- lutants may act in concert with temperature and relative hu- midity. New analytical methods are required to measure the oxi- dative and reactive species or specific markers thereof in indoor air.

Are We Measuring the Relevant Indoor Pollutants

We have developed a modeling and measurement framework for assessing transport of contaminated soils and airborne particulates into a residence, their subsequent distribution indoors via resuspension and deposition processes, and removal by cleaning and building exhalation of suspended particles. The model explicitly accounts for the formation of house dust as a mixture of organic matter (OM) such as shed skin cells and organic fibers, soil tracked-in on footwear, and particulate matter (PM) derived from the infiltration of outdoor air. We derived formulas for use with measurements of inorganic contaminants, crustal tracers, OM, and PM to quantify selected transport parameters. Application of the model to residences in the U.S. Midwest indicates that As in ambient air can account for nearly 60% of the As input to floor dust, with soil track-in representing the remainder. Historic data on Pb contamination in Sacramento, CA, were used to reconstruct sources of Pb in indoor dust, showing that airborne Pb was likely the dominant source in the early 1980s. However, as airborne Pb levels declined due to the phase-out of leaded gasoline, soil resuspension and track-in eventually became the primary sources of Pb in house dust.

Migration of Contaminated Soil and Airborne Particulates to Indoor Dust

Adsorption by dust samples from homes was studied to evaluate the extent to which household dust particles could act as carriers of vapors into the lower respiratory tract. The dust samples were examined for volatilizable and combustible materials via thermogravimetric analysis as well as for specific surface area and surface characteristics by nitrogen and formaldehyde adsorption and desorption isotherms. Particle morphology and size were examined by scanning electron microscopy (SEM) and elemental composition by energy dispersive x-ray analysis (EDXA). Thermogravimetric analysis (TGA) demonstrated marked differences between samples from different homes. The volatilizable material removed by heating dust samples in argon to 500°C ranged from 32% to 69%. The residue not combustible in oxygen at 700°C, presumably minerals, ranged from 11% to 58%. In most cases, differences between samples from the same borne were small. SEM of the samples demonstrated the presence of many fibers and of irregularly shaped p...

Paul A. Nagy

Papers

Surface Area, Adsorption, and Desorption Studies on Indoor Dust Samples