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W R Ott

Publications and source records attributed to W R Ott.

26 records · Page 2Linked to original sources
PubMedDate not supplied

Investigations of the proximity effect for pollutants in the indoor environment.

More than a dozen indoor air quality studies have reported a large discrepancy between concentrations measured by stationary indoor monitors (SIMs) and personal exposure monitors (PEMs). One possible cause of this discrepancy is a source proximity effect, in which pollutant sources close to the respondent cause elevated and highly variable exposures. This paper describes three sets of experiments in a home using real-time measurements to characterize and quantify the proximity effect relative to a fixed distant location analogous to a SIM. In the first set of experiments, using sulfur hexafluoride (SF6) as a continuously emitting tracer pollutant from a point source, measurements of pollutant concentrations were made at different distances from the source under different air exchange rates and source strengths. A second set of experiments used a continuous point source of carbon monoxide (CO) tracer pollutant and an array of high time resolution monitors to collect simultaneous concentration readings at different locations in the room. A third set of experiments measured particle count density and particle-bound polycyclic aromatic hydrocarbon (PAH) concentrations emitted from a continuous particle point source (an incense stick) using two particle counters and two PAH monitors, and included human activity periods both before and during the source emission period. Results from the SF6 and CO experiments show that while the source is emitting, a source proximity effect can be seen in the increases in the mean and median and in the variability of concentrations closest to the source, even at a distance of 2.0 m from the source under certain settings of air exchange rate and source strength. CO concentrations at locations near the source were found to be higher and more variable than the predictions of the mass balance model. For particles emitted from the incense source, a source proximity effect was evident for the fine particle sizes (0.3 to 2.5 microm) and particle-bound PAH up to at least 1.0 m from the source. Analysis of spatial and temporal patterns in the data for the three tracer pollutants reveal marked transient elevations of concentrations as seen by the monitor, referred to as "microplumes," particularly at locations close to the source. Mixing patterns in the room show complex patterns and directional effects, as evidenced by the variable intensity of the microplume activity at different locations. By characterizing the spatial and temporal variability of pollutant concentrations in the home, the proximity effect can be quantified, leading to improved indoor monitoring designs and models of human exposure to air pollutants.

Air Pollutants↗
PubMedDate not supplied

The effect of cigar smoking on indoor levels of carbon monoxide and particles.

To provide new information on environmental tobacco smoke (ETS) levels from cigars, we conducted three types of experiments: (1) Measurements of carbon monoxide (CO) during 15 controlled experiments in an office where several cigar brands were machine-smoked; (2) Measurements of CO or respirable suspended particles (RSP) and particle-bound polycyclic aromatic hydrocarbons (PAH) in a residence where two cigars were smoked by a person; and (3) Measurements of CO during two studies at cigar social events (where there were up to 18 cigars being smoked at a time) in which an investigator wore a concealed personal exposure monitor. Average concentrations of CO at the cigar social events were comparable to, or larger than, those observed on a freeway during rush hour traffic. A mass balance model that has been used successfully to predict ETS from cigarettes is used in this paper to obtain CO, RSP, and PAH emission factors (emission rate [mg/min], total mass emitted [mg], and emissions per mass smoked [mg/g]). The calculated emission factors show that the cigar can be a stronger source of CO than the cigarette. In contrast, the cigar may have fewer emissions of RSP and PAH per gram of consumed tobacco than the cigarette, but its size and longer smoking time results in greater total RSP and PAH emissions than for a single cigarette.

Air Pollutants↗
PubMedDate not supplied

The National Human Activity Pattern Survey (NHAPS): a resource for assessing exposure to environmental pollutants.

Because human activities impact the timing, location, and degree of pollutant exposure, they play a key role in explaining exposure variation. This fact has motivated the collection of activity pattern data for their specific use in exposure assessments. The largest of these recent efforts is the National Human Activity Pattern Survey (NHAPS), a 2-year probability-based telephone survey (n=9386) of exposure-related human activities in the United States (U.S.) sponsored by the U.S. Environmental Protection Agency (EPA). The primary purpose of NHAPS was to provide comprehensive and current exposure information over broad geographical and temporal scales, particularly for use in probabilistic population exposure models. NHAPS was conducted on a virtually daily basis from late September 1992 through September 1994 by the University of Maryland's Survey Research Center using a computer-assisted telephone interview instrument (CATI) to collect 24-h retrospective diaries and answers to a number of personal and exposure-related questions from each respondent. The resulting diary records contain beginning and ending times for each distinct combination of location and activity occurring on the diary day (i.e., each microenvironment). Between 340 and 1713 respondents of all ages were interviewed in each of the 10 EPA regions across the 48 contiguous states. Interviews were completed in 63% of the households contacted. NHAPS respondents reported spending an average of 87% of their time in enclosed buildings and about 6% of their time in enclosed vehicles. These proportions are fairly constant across the various regions of the U.S. and Canada and for the California population between the late 1980s, when the California Air Resources Board (CARB) sponsored a state-wide activity pattern study, and the mid-1990s, when NHAPS was conducted. However, the number of people exposed to environmental tobacco smoke (ETS) in California seems to have decreased over the same time period, where exposure is determined by the reported time spent with a smoker. In both California and the entire nation, the most time spent exposed to ETS was reported to take place in residential locations.

Adolescent↗
PubMedDate not supplied

Comparison of microenvironmental CO concentrations in two cities for human exposure modeling.

The microenvironmental components of the CO concentration in two cities are compared by subtracting the ambient background concentration from personal exposures measured in Denver, Colorado, and Washington, DC. Two surrogate measures for the ambient background concentration are tested. Both improve the similarity of the means in the two cities, but the Denver standard deviations are higher than those in Washington, DC. Microenvironments containing the internal combustion engine have both higher means and standard deviations in Denver compared with Washington, DC. The Washington, DC, mean concentration for automobiles, for example, was 59% of the Denver mean (2.0 ppm versus 4.9 ppm). Washington, DC, had approximately 57% of the Denver emissions, and the difference in mean CO concentrations is roughly consistent with the lower emissions in Washington, DC, due to lower elevation. A surprising finding is that mean CO exposure levels caused by cooking with gas stoves in Washington, DC, were only 58% of the levels in Denver (1.9 ppm and 3.3 ppm, respectively). This result suggests that elevation may exert an influence on gas stove emissions that is similar to its influence on internal combustion engines. Using an averaging time model, analysis of the autocorrelation of sleeping and office microenvironments suggests that considerable serial dependency exists. The microenvironmental data and findings in this paper have important implications for constructing human exposure-activity pattern models. For future human exposure field studies, the findings emphasize the importance of measuring background values in a location that is extremely close to each microenvironment studied.

Adult↗
PubMedDate not supplied

Residential air exchange rates for use in indoor air and exposure modeling studies.

Data on air exchange rates are important inputs to indoor air quality models. Indoor air models, in turn, are incorporated into the structure of total human exposure models. Fragmentary data on residential ventilation rates are available in various governmental reports, journal articles, and contractor reports. Most of the published papers present data on only a few homes to answer very specialized questions, and none of these publications summarize the ventilation rates of a large population of homes across the United States. Brookhaven National Laboratory (BNL) has conducted more than 4000 residential perfluorocarbon tracer (PFT) measurements and brought them together into a large data base from about 100 studies in the United States and elsewhere. This paper analyzes the BNL PFT data base to generate frequency distributions and summary statistics for different regions of the United States, different seasons, and different levels within the homes. The data analyses suggest that residential ventilation rates are similar in the northeastern and northwestern states but higher in the southwestern states. Winter and fall ventilation rates are similar, but the rates are slightly higher in spring, and much higher in summer. Multi-level residences have higher air exchange rates than single-level residences. Although the BNL data are not a representative sample of homes in the United States, these analyses give insight into the range of air exchange rates found in the United States under a great variety of conditions and are intended for use by developers of models of indoor air quality and total human exposure.

Air Pollution, Indoor↗
PubMedDate not supplied

Human exposure assessment: the birth of a new science.

I am honored to have been selected to receive the 1995 Weselowski Award for Career Achievement in Exposure Assessment. When I think back to my friendship with Jerry Weselowski, I remember our discussions in 1991 about the need to define exposure rigorously and quantitatively. In fact, I sent Jerry a letter on that topic--the quantitative definition of exposure--on November 7, 1991. I think it is fitting, therefore, that I select one of the topics in my talk today from that letter. Jerry always welcomed full and open scientific debate and discussion, and I feel that the need to define exposure and to understand what it means is a topic of great importance in exposure assessment. I also want to offer some observations about the importance of our field for the public and the regulatory community and to suggest the directions that exposure assessment might take as it evolves as a profession. Finally, I hope to provide an up-to-date listing of some selected scientific papers that are relevant to our field.

Adult↗
PubMedDate not supplied

A quantitative definition of exposure and related concepts.

This paper develops a unified theoretical framework for understanding exposure to environmental pollutants and other agents. It reviews the scientific literature to describe the many diverse and often confusing ways in which the term "exposure" is being used. Using six criteria proposed for a useful framework, a set of quantitative definitions, which encompass and expand upon existing definitions, is developed. After "agent" (e.g., a pollutant) and "target" (e.g., a person's hand) are defined, "exposure" is defined as the contact between an agent and a target. An "instantaneous point exposure" is defined as the joint occurrence of two events: 1) point i of a target is located at (xi, yi, zi) at time t, and 2) an agent of concentration Ci is present at location (xi, yi, zi) at time t. It is shown that the definition of instantaneous point exposure is fundamental in that all other functions of exposure with respect to space or time-such as the average exposure and the integrated exposure-can be derived from it. Because exposure and dose are closely related and often confused, our framework also includes a general definition of dose that is consistent with common usage. Finally, the definitions in this unified theoretical framework are shown to apply to inhalation exposure, dermal exposure, and ingestion exposure. In addition to the literature review and the quantitative definitions of exposure, this paper includes a glossary of terms that are proposed to help establish a common language for the exposure sciences.

Carbon Monoxide↗