Thomas H. Stock

Influence of ambient (outdoor) sources on residential indoor and personal PM2.5 concentrations: Analyses of RIOPA data

The Relationship of Indoor, Outdoor and Personal Air (RIOPA) study was designed to investigate residential indoor, outdoor and personal exposures to several classes of air pollutants, including volatile organic compounds, carbonyls and fine particles (PM2.5). Samples were collected from summer, 1999 to spring, 2001 in Houston (TX), Los Angeles (CA) and Elizabeth (NJ). Indoor, outdoor and personal PM2.5 samples were collected at 212 nonsmoking residences, 162 of which were sampled twice. Some homes were chosen due to close proximity to ambient sources of one or more target analytes, while others were farther from sources. Median indoor, outdoor and personal PM2.5 mass concentrations for these three sites were 14.4, 15.5 and 31.4 μg/m3, respectively. The contributions of ambient (outdoor) and nonambient sources to indoor and personal concentrations were quantified using a single compartment box model with measured air exchange rate and a random component superposition (RCS) statistical model. The median contribution of ambient sources to indoor PM2.5 concentrations using the mass balance approach was estimated to be 56% for all study homes (63%, 52% and 33% for California, New Jersey and Texas study homes, respectively). Reasonable variations in model assumptions alter median ambient contributions by less than 20%. The mean of the distribution of ambient contributions across study homes agreed well for the mass balance and RCS models, but the distribution was somewhat broader when calculated using the mass balance model with measured air exchange rates.

Fine organic particulate matter dominates indoor-generated PM2.5 in RIOPA homes

Residential indoor and outdoor fine particle (PM2.5) organic (OC) and elemental carbon (EC) concentrations (48u2009h) were measured at 173 homes in Houston, TX, Los Angeles County, CA, and Elizabeth, NJ as part of the Relationship of Indoor, Outdoor and Personal Air (RIOPA) study. The adsorption of organic vapors on the quartz fiber sampling filter (a positive artifact) was substantial indoors and out, accounting for 36% and 37% of measured OC at the median indoor (8.2u2009μgu2009C/m3) and outdoor (5.0u2009μgu2009C/m3) OC concentrations, respectively. Uncorrected, adsorption artifacts would lead to substantial overestimation of particulate OC both indoors and outdoors. After artifact correction, the mean particulate organic matter (OM=1.4 OC) concentration indoors (9.8u2009μg/m3) was twice the mean outdoor concentration (4.9u2009μg/m3). The mean EC concentration was 1.1u2009μg/m3 both indoors and outdoors. OM accounted for 29%, 30% and 29% of PM2.5 mass outdoors and 48%, 55% and 61% of indoor PM2.5 mass in Los Angeles Co., Elizabeth and Houston study homes, respectively. Indirect evidence provided by species mass balance results suggests that PM2.5 nitrate (not measured) was largely lost during outdoor-to-indoor transport, as reported by Lunden et al. This results in dramatic changes with outdoor-to-indoor transport in the mass and composition of ambient-generated PM2.5 at California homes. On average, 71% to 76% of indoor OM was emitted or formed indoors, calculated by (1) Random Component Superposition (RCS) model and (2) non-linear fit of OC and air exchange rate data to the mass balance model. Assuming that all particles penetrate indoors (P=1) and there is no particle loss indoors (k=0), a lower bound estimate of 41% of indoor OM was indoor-generated (mean). OM appears to be the predominant species in indoor-generated PM2.5, based on species mass balance results. Particulate OM emitted or formed indoors is substantial enough to alter the concentration, composition and behavior of indoor PM2.5. One interesting effect of increased indoor OM concentrations is a shift in the gas-particle partitioning of polycyclic aromatic hydrocarbons (PAHs) from the gas to the particle phase with outdoor-to-indoor transport.

The Estimation of Personal Exposures to Air Pollutants for a Community-Based Study of Health Effects in Asthmatics—Design and Results of Air Monitoring

In order to provide reliable pollutant and meteorological exposure estimates for an epidemiological study of asthmatics residing in two Houston neighborhoods, a dedicated three-tier air monitoring system was established. This consisted of fixed site ambient air monitoring at the center of each study area, a mobile van performing simultaneous indoor and outdoor measurements at selected residences of study participants, and a limited amount of direct personal monitoring for half of the participants. Monitored pollutants Included all criteria pollutant gases, as well as aeroallergens, aldehydes, TSP, and IP. Laboratory analyses provided concentrations of sulfate, nitrate, and trace elements. Continuous measurements of several meteorological parameters also were obtained. Intensive quality assurance and data validation efforts resulted in a high percentage of valid data for most pollutants. Ozone was the only measured pollutant that exceeded the NAAQS during the six-month (May to October) study period. The mo...

A Survey of Typical Exposures to Formaldehyde in Houston Area Residences

A survey of indoor air quality under warm weather conditions, in a variety of Houston area residences not selected in response to occupant complaints, revealed a distribution of indoor formaldehyde concentrations ranging from less than 0.008 ppm to 0.29 ppm, with an arithmetic mean of 0.07 ppm. Approximately 15% of the monitored residences had concentrations greater than 0.10 ppm. Formaldehyde levels were observed to depend on both age of dwelling and the structural classification of the residence. These factors are not independent and reflect the influence of more fundamental variables, such as the rate of exchange of indoor and outdoor air and the overall emission potential of indoor materials. The results of this survey suggest that considerable population exposures to excess (greater than 0.10 ppm) formaldehyde concentrations may occur in the residential environment, indicating the need for improved control strategies.

Formaldehyde Concentrations Inside Conventional Housing

As part of the exposure assessment scheme for a community-based air pollution health effects study, 43 homes of study participants, located in two Houston neighborhoods, were monitored for weekly-average indoor formaldehyde levels by means of diffusion samplers. Consecutive 12-hour aldehyde sampling for one-week periods was conducted in 12 of the homes by means of pumps and impingers. In six houses where simultaneous monitoring with both methods occurred, good correlation between the results from the diffusion samplers and the standard impinger method was observed. Diffusion sampler precision was variable and lower than expected, and a small positive measurement bias could be inferred. The distribution of house-average indoor formaldehyde concentrations from diffusion monitoring was similar to that obtained during a previous housing survey in Houston, with concentrations in 19% of the homes exceeding 0.10 ppm. Formaldehyde levels in this group of conventional, mostly older homes could not be associated with smoking, cooking, home age or structure type. However, there was a statistically significant difference between mean indoor concentrations in the two neighborhoods.

Inhalation exposures to acrylamide in biomedical laboratories

This study evaluated airborne acrylamide exposures experienced by laboratory personnel using either crystalline or commercially available solutions of acrylamide to make polyacrylamide gels. Exposures were monitored for a short-term (15-min) sampling period, during the weighing of the crystalline acrylamide or the removal of the acrylamide solution from its original container, and a long-term period, during which a sample was collected for as long as the subject was potentially exposed to acrylamide. Mean air concentrations for the 15-min exposures were 7.20 +/- 5.64 micrograms/m3 and 5.81 +/- 4.53 micrograms/m3 for the users of crystalline and solution acrylamide, respectively, although this difference was not statistically significant (p > 0.05). Mean concentrations for the long-term exposures were 12.77 +/- 24.20 micrograms/m3 for workers employing crystalline acrylamide and 4.22 +/- 7.05 micrograms/m3 for personnel using acrylamide solutions. This difference was also not statistically significant. Although the results indicate that the research laboratory personnel were generally exposed to measurable concentrations of acrylamide, with several subjects exposed to elevated levels, the calculated 8-hour time-weighted average exposures were below current occupational exposure limits. However, because the neurotoxic effects of acrylamide are cumulative and it is a suspected carcinogen, all exposures should be kept as low as reasonably achievable.

The Estimation of Personal Exposures to Air Pollutants for a Community-Based Study of Health Effects in Asthmatics—Exposure Model

Estimates of individual personal exposure to ozone, nitrogen dioxide, pollen, temperature, and relative humidity for a group of asthmatics participating in a health effects study were obtained by means of a modelling approach utilizing fixed site monitoring data, regression relationships between fixed site and indoor and outdoor microenvironment concentrations, study subject activity patterns and study household characteristics. A considerable improvement in the accuracy of exposure assessment using the exposure model instead of fixed site measurements alone was demonstrated for ozone. This large refinement of zone exposure estimates was achieved using a simplified approach which emphasized the large differences between indoor and outdoor microenvironmental concentrations, and assumed relatively little heterogeneity in exposure within either of these two broad microenvironmental categories. Major sources of error in the exposure model for ozone include: failure to include indoor microenvironments with no air conditioning in the development of the model, inability to accurately apportion within-hour time spent in different microenvironments, and misclassification of hour-specific personal location by study subjects.

An Evaluation of the Effect of Source and Concentration on Three Methods for the Measurement of Formaldehyde in Indoor Air

Three methods for measuring formaldehyde (HCHO) in indoor air were evaluated under field and laboratory conditions using different sources and concentrations of formaldehyde in air. Two impinger methods (the chromotropic acid method and modified pararosaniline method) and the Draeger short-term detector tube method (with and without activation tubes) were compared when sampling for formaldehyde from a particle board box, formalin solution, and a conventional home. Concentrations of formaldehyde ranged from 0.05-0.5 ppm in air. All samples were collected independently using personal sampling pumps and a Draeger bellows pump. The results indicate that the Draeger tube method using an activation tube gives lower results than either of the impinger methods. Without using an activation tube (concentrations greater than 0.5 ppm), the Draeger tube method was comparable to the two impinger methods. In addition, there are indications that the chromotropic acid method gives different results than the modified pararosaniline method, depending on the source of formaldehyde. The modified pararosaniline method indicated higher results than the chromotropic acid method when sampling from a particle board++ box but not from a formalin source. Overall analytical precision for each method of analysis was good.

The use of detector tube humidity limits

The current methods employed by detector tube manufacturers to report sampling limitations imposed by ambient humidity conditions may be confusing and sometimes misleading to the user. Absolute humidity limits (mg/L of water) are inconvenient to use and are often ignored, even though they frequently impose surprisingly restrictive sampling conditions. A short microcomputer program to convert absolute humidity limits into temperature-dependent relative humidity limits is presented. Temperature-independent relative humidity limits are easier to use but the scientific basis of such limits is questionable. Several recommendations are presented for improving the usefulness and reliability of detector tube humidity limits.

The nonuniform application of TLV's as legal workplace standards

A survey of current Federal exposure limits for workplace air contaminants has revealed a surprising diversity of applicable TLVs. At least six different TLV lists are enforceable as legal standards by the Department of Labor. This nonuniformity is contrary to legislative intent, and is a source of administrative difficulty as well as inequity in worker protection.