Ozone Excerpts 4

Excerpts Taken From

Integrated Science Assessment for Ozone and
Related Photochemical Oxidants

U.S. Environmental Protection Agency
September 2011

Excerpted by Michael H. Klein

Page 4-1 line 20 (ARP 359)

A theoretical model of personal exposure is presented to highlight measurable quantities and the uncertainties that exist in this framework. An individual’s time-integrated total exposure to O3 can be described based on a compartmentalization of the person’s activities throughout a given time period:

Equation 4-1


      ET = total (T) exposure over a time-period of interest
      Cj = airborne O3 concentration at microenvironment j
      dt = portion of the time-period spent in microenvironment j

Equation 4-1 can be decomposed into a model that accounts for exposure to O3, of ambient (Ea) and nonambient (Ena) origin of the form:

Equation 4-2

Ambient O3 is formed through photochemical reactions involving NOX, VOCs, and other compounds, as described in Chapter 3. Although nonambient sources of O3 exist, such as O3 generators and laser printers, these sources are specific to individuals and may not represent important sources of population exposure. Ozone concentrations generated by ambient and nonambient sources are subject to spatial and temporal variability that can affect estimates of exposure and influence epidemiologic effect estimates.

Page 4-2 line 11 (ARP 360)

This assessment focuses on the ambient component of exposure because this is more relevant to the NAAQS review. Assuming steady-state outdoor conditions, Ea can be expressed in terms of the fraction of time spent in various outdoor and indoor microenvironments

Equation 4-3


      f = fraction of the relevant time period (equivalent to dt in Equation 4-1)
      subscript o = index of outdoor microenvironments
      subscript i = index of indoor microenvironments
      subscript o,i = index of outdoor microenvironments adjacent to a given indoor microenvironment i
      Finf,i = infiltration factor for indoor microenvironment i

Equation 4-3 is subject to the constraint Sfo + Sfi = 1 to reflect the total exposure over a specified time period, and each term on the right hand side of the equation has a summation because it reflects various microenvironmental exposures. Here, “indoors” refers to being inside any aspect of the built environment, e.g., home, office buildings, enclosed vehicles (automobiles, trains, buses), and/or recreational facilities (movies, restaurants, bars). “Outdoor” exposure can occur in parks or yards, on sidewalks, and on bicycles or motorcycles. Finf is a function of the building air exchange characteristics. Assuming steady state ventilation conditions, the infiltration factor is a function of the penetration (P) of O3 into the microenvironment, the air exchange rate (a) of the microenvironment, and the rate of O3 loss (k) in the microenvironment:

Finf = Pa/(a+k)

In epidemiologic studies, the central-site ambient concentration, Ca, is often used in lieu of outdoor microenvironmental data to represent these exposures based on the availability of data. Thus it is often assumed that Co = Ca and that the fraction of time spent outdoors can be expressed cumulatively as fo; the indoor terms still retain a summation because infiltration differs among different microenvironments. If an epidemiologic study employs only Ca, then the assumed model of an individual’s exposure to ambient O3, first given in Equation 4-3, is re-expressed solely as a function of Ca:

Equation 4-4

The spatial variability of outdoor O3 concentrations due to meteorology, varying precursor emissions and O3 formation rates; design of the epidemiologic study; and other factors determine whether or not Equation 4-4 is a reasonable approximation for Equation 4-3. Errors and uncertainties inherent in use of Equation 4-4 in lieu of Equation 4-3 are described in Section 4.6 with respect to implications for interpreting epidemiologic studies. Epidemiologic studies often use concentration measured at a central site monitor to represent ambient concentration; thus a, the ratio between personal exposure to ambient O3 and the ambient concentration of O3, is defined as:

Equation 4-5

Combination of Equation 4-4 and Equation 4-5 yields:

Equation 4-6

where a varies between 0 and 1. If a person’s exposure occurs in a single microenvironment, the ambient component of a microenvironmental O3 concentration can be represented as the product of the ambient concentration and Finf. Wallace et al. note that time-activity data and corresponding estimates of F3 concentration can be represented as the product of the ambient concentration and Finf. Wallace et al. note that time-activity data and corresponding estimates of Finf for each microenvironmental exposure are needed to compute an individual’s a with accuracy. In epidemiologic studies, a is assumed to be constant in lieu of time-activity data and estimates of Finf, which can vary with building and meteorology-related air exchange characteristics. If local outdoor sources and sinks exist and are significant but not captured by central site monitors, then the ambient component of the local outdoor concentration may be estimated using dispersion models, land use regression models, receptor models, fine scale CTMs or some combination of these techniques. These techniques are described in Section 4.5.

Page 4-5 line 7 (ARP 363)

Several studies summarized in the 2006 O3 AQCD, along with some newer studies, have evaluated the relationship between indoor O3 concentration and the O3 concentration immediately outside the indoor microenvironment. These studies show that the indoor concentration is often substantially lower than the outdoor concentration unless indoor sources are present. Low indoor O3 concentrations can be explained by reactions of O3 with surfaces and airborne constituents. Studies have shown that O3 is deposited onto indoor surfaces where reactions produce secondary pollutants such as formaldehyde. However, the indoor-outdoor relationship is greatly affected by the air exchange rate; under conditions of high air exchange rate, such as open windows, the indoor O3 concentration may approach the outdoor concentration… In general, I/O ratios range from about 0.1 to 0.4, with some evidence for higher ratios during the O3 season when concentrations are higher.

Page 4-8 line 7 (ARP 366)

Several factors influence the relationship between personal O3 exposure and ambient concentration. Due to the lack of indoor O3 sources, along with reduction of ambient O3 that penetrates into enclosed microenvironments, indoor and in-vehicle O3 concentrations are highly dependent on air exchange rate and therefore vary widely in different microenvironments. Ambient O3 varies spatially due to reactions with other atmospheric species, especially near busy roadways where O3 concentrations are decreased by reaction with NO (Section This is in contrast with pollutants such as CO and NOX, which show appreciably higher concentrations near the roadway than several hundred meters away. O3 also varies temporally over multiple scales, with a generally increasing trend during the daytime hours, and higher O3 concentrations during summer than in winter. An example of this variability is shown in Figure 4-1, taken from a personal exposure study conducted by Chang et al.

Hourly personal exposures are seen to vary from a few ppb in some indoor microenvironments to tens of ppb in vehicle and outdoor microenvironments. The increase in ambient O3 concentration during the day is apparent from the outdoor monitoring data. In comparison, ambient PM2.5 exhibits less temporal variability over the day than O3, although personal exposure to PM2.5 also varies by microenvironment. This combined spatial and temporal variability for O3 results in varying relationships between personal exposure and ambient concentration.

Page 4-10 line 19 (ARP 368)

The results of these studies indicate that personal exposures are moderately well correlated with ambient concentrations, and that the ratio of personal exposure to ambient concentration is higher in outdoor microenvironments and during the summer season. In situations where a lack of correlation was observed, this may be due in part to a high proportion of personal measurements below the detection limit. The effect of season is unclear, with mixed evidence on whether higher correlations are observed during the O3 season.

Page 4-14 line 5 (ARP 372)

Exposure to ambient O3 occurs in conjunction with exposure to a complex mixture of ambient pollutants that varies over space and time. Multipollutant exposure is an important consideration in evaluating health effects of O3 since these other pollutants have either known or potential health effects that may impact health outcomes due to O3. The co-occurrence of high O3 concentrations with high heat and humidity may also contribute to health effects. This section presents data on relationships between overall personal O3 exposure and exposure to other ambient pollutants, as well as co-exposure relationships for near-road O3 exposure.

Page 4-19 line 16 (ARP 377)

Individuals can reduce their exposure to O3 by altering their behaviors, such as reducing their time outdoors. To protect the public from O3-related health effects, EPA and organizations such as the American Lung Association recommend that people spend more time indoors and engage in less strenuous activities on days with relatively high O3 concentrations. To assist individuals concerned about O3 conditions, EPA developed the Air Quality Index (AQI). This index combines information about O3 (and other pollutant) concentrations to produce five categories of air-quality, ranging from good to very unhealthy. Forecasted and actual conditions typically are reported to the public during high-O3 months through local media outlets, using various versions of this air-quality categorization scheme. These advisories explicitly state that children in general and children with asthma in particular are potentially sensitive to air pollution. Parents are advised to curtail children’s outdoor exertion to varying degrees depending on the predicted pollution levels and whether their children have asthma or other relevant medical conditions.

Page 4-33 line 12 (ARP 391)

For other criteria pollutants, nonambient sources can be an important contributor to total personal exposure. There are relatively few indoor sources of O3; as a result, personal O3 exposure is expected to be dominated by ambient O3 in outdoor microenvironments and in indoor microenvironments with high air exchange rates (e.g., with open windows). Even in microenvironments where nonambient exposure is substantial, such as in a room with an O3 generator, this nonambient exposure is unlikely to be temporally correlated with ambient O3 exposure, and therefore would not affect epidemiologic associations between O3 and a health effect. In simulations of a nonreactive pollutant, Sheppard et al. concluded that nonambient exposure does not influence the health outcome effect estimate if ambient and nonambient concentrations are independent. Since personal exposure to ambient O3 is some fraction of the ambient concentration, it should be noted that effect estimates calculated based on personal exposure rather than ambient concentration will be increased in proportion to the ratio of ambient concentration to ambient exposure, and daily fluctuations in this ratio can widen the confidence intervals in the ambient concentration effect estimate, but uncorrelated nonambient exposure will not bias the effect estimate.

Page 4-37 line 21 (ARP 395)

The relationship between personal exposure and ambient concentration has been found to vary by season, with at least three factors potentially contributing to this variation: differences in building ventilation (e.g., air conditioning or heater use versus open window ventilation), higher O3 concentrations during the O3 season contributing to increased exposure and improved detection by personal monitors; and changes in activity pattern resulting in more time spent outside. Evidence has been presented in studies conducted in several cities regarding the effect of ventilation on personal-ambient and indoor-outdoor O3 relationships (see Sections 4.3.2 and 4.3.3). More limited evidence is available regarding the specific effects of O3 detection limits and activity pattern changes on O3 relationships.

Page 10-4 line 4 (ARP 1092)

The impact on climate of the tropospheric O3 change since preindustrial times has been estimated to be about 25-40% of anthropogenic CO2 impact and about 75% of anthropogenic CH4 impact, ranking it third in importance of the greenhouse gases. In the 21st century as the Earth’s population continues to grow and energy technology spreads to developing countries, a further rise in the global concentration of tropospheric O3 is likely, with associated consequences for human health and ecosystems relating to climate change.

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