Mean Concn. Microenvironment ppm Time min b ppm Indoors, parking garage In transit, car In transit, other vehicle Outdoors, near roadway In transit, walking Indoors, restaurant Indoors, office Indoors, store Indoors, residence 19 8 8 4 4 4 3 3 2 14 71 66 33 28 58 50 11 s 4 3 2 2 2 3 1 a Mean concentration in the specific microenvironment during the time that subjects were there. Time min b 11 79 49 20 32 45 36 1, Gas Outdoors 9 5 1.
Personal exposure to these chemicals was consistently higher than re- corded outdoor concentrations, sometimes by an order of magnitude. Concentrations of these same compounds in exhaled breath correlated with personal exposure measurements Wallace et al. Personal exposure for Pb is not well represented by outdoor measure- ments, since exposures occur primarily during commuting periods Fugas et al.
Air Quality Monitoring Measuring And Modeling Environmental Hazards
Highest Pb levels are encountered near streets with heavy traffic patterns. Therefore, individuals who spend a great deal of time in associated microenviron- ments, such as taxi drivers, experience the highest exposures Azar et al. Few personal monitoring stud- ies have been undertaken for O3. Available Table 7. Sexton and Ryan evidence suggests that outdoor O3 values significantly overestimate personal expo- sure for most people because so much time is typically spent indoors where levels are substantially lower Stock et al.
Indirect Approach to Exposure Estima- tion. Given the diversity of microenvi- ronments that people move through each day see table 2 , application of the indirect approach to exposure assessment is not straightforward. Its utility depends on identification of and sampling in the micro- environments with the greatest potential to influence human exposure. The costs and practical difficulties of monitoring in all, or even most, of the locations where people are likely to spend their time limits the scope of indirect measurements.
Most studies are designed in conjunction with direct measurements and typically include sampling only of community outdoor air and of air inside private residences. Most air pollution measurements have been made in the outdoor community air microenvironment.
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Nevertheless, because most people are outside for such a small fraction of the time, and because the amount of air pollution that penetrates in- doors is modified by building characteris- tics, outdoor measurements are of marginal value in estimating the actual exposures of humans to many airborne contaminants. Information on important microenviron- ments and sources that affect exposure to air pollutants is given in table 8.
In-home concentrations of NO2 Sexton et al. Outdoor or in-vehicle concen- trations in areas of heavy traffic are the primary determinants of personal exposure for CO Ott and Flachsbart ; Nagda and Koontz and Pb Azar et al. Nonoccupational exposures to formaldehyde are likely to occur primar- ily indoors, especially in newer manufac- tured residences, because of emissions from building materials and furnishings Envi- ronmental Health Perspective ; Sexton et al. Research Recommendation Exposure Monitoring.
Past exposure as- sessment studies. Participants were often volunteers willing to undergo the in- convenience of personal or indoor monitor- ing. In only a few studies was a statistically random sample of participants selected Akland et al. These exploratory studies indicated that fixed-site, outdoor monitors do not ade- quately estimate human exposure to most air pollutants, and that accurate extrapola- tion to people beyond the sample popula- tion often is not possible. Furthermore, the high degree of variability in pollutant con- centrations within individual buildings, within neighborhoods, and within cities will require substantial work to determine the distribution of exposures across a pop- ulation for example, frequency distribu- tion for a city.
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In general, exposure assess- ment has not been not undertaken as part of a comprehensive and coordinated effort to define exposure distributions for a specific group of people. Exposure Moni- toring. Studies are required to provide data on human exposures and to investigate the link between measured exposures and adverse health effects. These efforts will require a the development of suitable in- struments for example, personal and in- door monitors and measurement tech- niques for example, noninvasive biological tests ; b the application of appropriate.
The respective roles of air monitoring direct and indirect approaches and biolog. Direct pollutant concentrations in a representative measurements of personal exposure are sample, and determine to what extent the straightforward but costly, time-con- indirect approach complements direct ex suming, and labor-intensive. Indirect ap- posure measurements. Moreover, direct measurements are essential to validate the results of indirect microenvironmental exposure studies.
Future studies should evaluate the utility of the indirect approach, which depends on the identification of the important micro- environments and the ability to measure.. Sexton and Ryan Both the direct and the indirect measure- ment schemes should be coupled with a biological monitoring program to investigate the relationship between air concentrations, biological exposure markers, and related health consequences.
The Changing Paradigm of Air Pollution Monitoring | Environmental Science & Technology
This linkage is critical for regulatory purposes, since the airborne levels of a specific contaminant are typically the focus of health-based regulations. Biological Monitoring Most of the biological data on exposure markers are related to occupational envi- ronments. Lead workers, for example, are tested routinely for clinical signs and symp- toms of Pb absorption, using markers such as porphyrins, B-aminolevulinic acid, and aminolevulinate dehyratase. Biological monitoring has not been used extensively outside the workplace.
Among the exceptions are the analysis of adipose tissue and other body compartments to determine residues for chlorinated hydro- carbons Hayes , of human milk to assess exposure to organochlorine pesti- cides Savage et al. Techniques for the biological monitoring of human exposure to carcinogenic and mutagenic agents are a relatively recent development Goldstein ; Berlin et al.
Examples of techniques that have been used to detect human exposures to carcinogenic materials include determina- tion of the degree of hemoglobin histidine alkylation as a measure of ethylene oxide exposure Osterman-Golkar et al. Research Recommendation Biological Markers of Exposure. Biolog- ical monitoring has evolved rapidly, espe- cially for applications in the industrial workplace. Recent advances suggest that available biological measurement tech- niques can be applied to a wider range of environmental contaminants, including community and indoor air pollution.
In addition, specific studies might be undertaken to identify and mon- itor potential exposure markers in surviv- ing animals throughout the course of large- scale chronic animal studies, to evaluate. Biological Mark- ers of Exposure. Research is required to adapt available biological measurement techniques to community air pollution studies.
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Studies that better define the nature of the relationship between exposure and dose, and between dose and health out- come, are needed. It is especially important to establish in controlled human popula- tions a the sources of error for a particular biological measurement technique, b the validity of sample collection methodology, c the appropriateness of internal and ex- ternal standards, and d the adequacy of methods for quality control. Modeling Human Exposure to Air Pollution Models are useful tools to quantify the relationship between air pollutant exposure and important explanatory variables for example, time-activity patterns , as well as to estimate exposures in situations where measurements are unavailable.
In the United States, there are approximately million people, about 82 million residences, and some 15 million commercial and public buildings. It is impractical to have everyone carry a personal monitor or to undertake pollutant measurements inside all build- ings. Exposure models obviate the need for such extensive measurement programs by providing estimates of population expo- sures that are based on a small number of representative measurements.
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The chal- lenge is to develop appropriate models that allow for extrapolation from relatively few exposure measurements to a much larger population. Human exposure to air pollution can be thought of as a physical system in the same. Newton's laws of motion might be selected as an appropriate model, for example, to de- scribe the motion of an object falling in a gravitational field.
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Subsequent to selecting this model, it is necessary to verify it through experimentation. Experimental data might indicate that certain important determinants, such as air resistance, have been left out of the model. Statistical models are those for which predicted values are approximations subject to uncertainties introduced by factors de- liberately omitted in the model, as well as by measurement errors. Techniques of sta- tistical data analysis a sophisticated form of curve-fitting may be used instead of modeling the contributing physical, chem- ical, and biological processes, even if the processes are known and understood.
For simplicity and clarity, the following discussion distinguishes between statistical exposure models developed from a stochas- tic or probabilistic perspective, and physi- cal exposure models, defined as those that are developed from an understanding of the underlying physical processes. This is an artificial distinction that focuses on the ori- entation, either statistical or physical, from which the model was constructed.
In truth, all exposure models are statistical to some degree because the physical laws that con- strain human activities and pollutant con- centrations do not take into account abso- lutely every contributing process and factor. Physical-stochastic models, which combine the concrete aspects of the physi- cal approach with the probabilistic aspects of the statistical approach, are also dis- cussed. All three classifications of exposure models are compared in table 9. Statistical Modeling The statistical approach requires the collec- tion of data on human exposures and the factors thought to be determinants of ex- posure.
These data are combined in a sta. Sexton and Ryan Table 9.
4.1 Using appropriate monitoring methods
Comparison of Different Approaches to Air Pollution Exposure Modeling Model Type Parameter Statistical Physical Physical-Stochastic Method of formulation Required input Collected data on human exposure Hypothesis testing Advantages Makes use of real data in the model-building process Disadvantages Physical laws Knowledge of important pa- rameters and their values in the system to be modeled True model developed from a.
In principle, the results are appli- cable only to the data used to produce the model. If the study population constitutes a representative sample, however, then ex- trapolation of results to a broader group may be justified. Spurious conclusions may be drawn, for example, from statistical models that include parameters that are correlated with, but not causally related to, air pollution exposure.
Initial statistical analysis of the data set usually focuses on defining the range of deviation associated with each variable and examining the bivariate and multivariate correlations between and among the inde- pendent variables. A lack of sufficient vari- ability for one or more of the parameters in the model will limit the development of a useful predictive tool, whereas inclusion of independent variables that are highly col- linear may obscure the importance of crit- ical exposure determinants.
Once this type of exploratory analysts is completed, model building can begin. One of the simplest statistical models is the categorical comparison, in which exposure data are compared for two or more catego- ries of a specific independent variable.