National Ambient Air Quality Objectives For Particulate Matter - Executive Summary

Human Exposure Assessment

The likelihood of an adverse response to particles is influenced by the degree of exposure, defined as any contact between a pollutant at a specified concentration and the outer (e.g., skin) or inner (e.g., respiratory tract epithelium) surface of the human body. Changes in the degree of exposure are influenced by the duration, magnitude and frequency of exposure. Inhalation is the only PM exposure pathway to considered in this assessment.

Ambient concentrations of particles are typically measured over a 24 hour sampling period, as described above. Over a 24 hour period, a person spends their time in many locations or microenvironments. For example, most people spend a great deal of time in indoor environments, at home and at work; some time each day in vehicles; and relatively little time each day outdoors. The proportion of time spent in different environments will vary with age, gender and day of the week. To the extent that microenvironmental PM concentrations are different from outdoor concentrations, population (and individual) exposures to PM will be different from those estimated from ambient monitoring data. The high correlations that have been found between personal exposure and indoor PM concentrations, combined with the amount of time spent indoors, indicate that indoor microenvironments are the most important contributors to PM exposure.

Indoor levels of particles are a function of: indoor sources, outdoor particle levels, the fraction of ambient air penetrating indoors, filtration, air exchange (e.g., older houses tend to be more leaky), particle decay and resuspension rates (e.g. from vacuuming or dusting). The latter source, the so-called "Pigpen" or "personal cloud" effect, helps explains why actual personal exposure is usually greater than indirect estimates combining indoor and outdoor concentrations and time-activity information. The increase in particle concentration as a result of a person occupying the microenvironment is overlooked.

Several early studies indicated that penetration of ambient air into indoor environments is more effective for fine particles than coarse particles. Some more recent studies have indicated that penetration factors for both fine and coarse particles are close to unity. Nonetheless, current scientific thinking maintains that small particles penetrate indoors more effectively than larger particles. In Canada, where building construction emphasizes energy efficiency, and therefore low air exchange rates, the fractions of fine and coarse particles of ambient origin that will be found indoors under equilibrium will tend toward 50% or less, particularly in the winter. Once inside, the larger particles tend to settle out more quickly than smaller particles; however, the larger particles are more easily resuspended as a result of indoor activities.

Cigarette smoking has been identified as the major source of indoor particles (particularly fine PM, but also PM₁₀) in smoking households, raising indoor PM concentrations significantly above those in non-smoking 3 households. A PM_2.5 concentration of 30 µg/m corresponds to the impact of smoking approximately one pack of cigarettes per day. In non-smoking households, outdoor air is the major source of indoor PM levels. Other indoor sources of particles include such things as wood burning and kerosene stoves and heaters, animal dander, home care and personal care products and various indoor sources of mineral fibres. In general, large home-to-home variations in indoor particle concentrations can be expected. Some studies have reported mean indoor concentrations greater than outdoor levels, while others conclude just the opposite. In many cases, the range of particle concentrations indoors and outdoors is similar. However, in areas where outdoor levels are fairly high, indoor concentrations may well be less, whereas indoor concentrations may greatly exceed outdoor concentrations in areas where outdoor levels are relatively low.

Correlations between ambient PM data obtained from fixed ambient monitors (FAMs) and personal exposure data obtained from PEMs have been explicitly examined in many studies. Most of these reveal poor correlations and show, not surprisingly, that personal exposures are usually greater than (either indoor or) outdoor ambient concentrations. Furthermore, most studies report poor cross-sectional personal-outdoor correlations. If there is a lot of variability in particle concentrations from sources poorly correlated with FAMs, then the percent of variance in personal exposures that can be explained by the FAM data is likely to be small. Sampling error, a non-random sample, very strong indoor sources and personal activities all contribute to a poor correlation between

FAMs and PEMs. However, for individuals who are not exposed to microenvironmental sources of particles (e.g. smoking), and whose day-to-day activities are fairly repetitive, ambient levels of particles may more directly reflect their exposure to particles. Introducing microenvironmental exposures into a time-weighted average exposure estimate will theoretically improve estimates of personal exposure to total PM. Studies have shown this to be the case; however, such indirect personal exposure estimates are still likely to underestimate actual personal exposures.

Personal and population exposure models have been developed that combine ambient measurements of pollutants with information on age-specific time-activity and estimates of microenvironmental pollutant concentrations. A probabilistic PM₁₀ exposure model was applied to Canadian data to produce estimates of distributions of 24 hour average personal, indoor, outdoor and in-transit PM₁₀ concentrations. Predicted mean 24 hour personal PM₁₀ exposure (across all regions and seasons) is 39 µg/m³ Median personal PM₁₀ exposure is predicted to be 31 µg/m³. Comparing these results with exposure predictions based on urban NAPS site data (i.e., ambient PM data) clearly shows that ambient data underestimate average population exposures to total PM (mean and median urban ambient PM₁₀ are approximately 28 µg/m³ and 24 µg/m³ respectively). Canadian estimates of exposure to PM_2.5 have not yet been predicted through exposure modelling. Given the current information, it is reasonable to conclude that the ambient data alone can represent the lower range in the distribution of total particle exposures. Further studies are required to improve various components of the exposure model, including better characterization of indoor PM sources and penetration rates of ambient particles indoors, specifically in cold climates, since air exchange rates are a function of ambient temperature.