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Researchers characterize aggregate indoor and outdoor PM2.5 exposure

In an initial step toward developing a comprehensive global impact assessment framework for PM2.5 emissions, an international team of researchers from the US and Europe has characterized the primary PM2.5 intake fraction—the long-term population intake mass per unit mass emitted into different indoor and outdoor environments.

Intake fractions from residential and occupational indoor sources ranged from 470 ppm to 62,000 ppm—mainly as a function of air exchange rate and occupancy. Indoor exposure typically contributes 80−90% to overall exposure from outdoor sources. In a paper in the ACS journal Environmental Science & Technology, the researchers suggested that their framework facilitates improvements in air pollution reduction strategies and life cycle impact assessments.

Over the last three decades, multiple epidemiological and toxicological studies have attributed a range of adverse health impacts including chronic and acute respiratory and cardiovascular diseases and premature mortality to exposures to fine particulate matter (PM2.5, representing particles with aerodynamic diameter of 2.5 μm or smaller) both outdoors and indoors.

In the Global Burden of Disease (GBD) study series, exposure to PM2.5 is identified as a leading environmental risk factor contributing to global human disease burden. PM2.5 in outdoor air and household air is reported to contribute to estimated 4.2 and 2.9 million premature deaths, respectively, corresponding to 103 and 86 million disability-adjusted life years (DALY), respectively, in 2015. Indoor and outdoor emissions of primary PM2.5 from anthropogenic sources contribute substantially to human exposures, which take place both indoors and outdoors.

… To inform decisions for comparing and reducing PM2.5 exposure from anthropogenic sources, a quantitative framework is required to link indoor and outdoor environments. … a consistently coupled indoor-outdoor exposure assessment framework is currently missing that allows for comparing PM2.5-related intake fractions from a range of human activities that lead to outdoor and indoor sources resulting in human exposures to PM2.5 both indoors and outdoors.

—Fantke et al.

Population-weighted distribution of effective intake fractions (kg PM2.5 inhaled per kg PM2.5 emitted) and contribution of indoor exposure (percent of total intake fraction) for residential and occupational indoor emission scenarios and for ground-level urban (range over all cities per region) and continental rural outdoor emission scenarios. Credit: ACS, Fantke et al. Click to enlarge.

In their study, the researchers first structured the PM2.5 emission-to-intake pathway into a system of archetypes representing a tiered approach with different levels of detail for indoor and urban- and rural-outdoor environments.

They described the system as a fully mass-balance-based framework for relating indoor and outdoor emissions to aggregate PM2.5 exposure.

They then analyzed the variations of intake fraction among different locations as a function of advection rates (in this case, the horizontal transport of matter by fluid flow in the atmosphere) and population densities, based on differentiating for each source scenario the contribution of each environment to the overall population exposure.

They proposed a system of archetypes at different levels of detail that provide a higher level of resolution than can be achieved with currently available spatial models. They defined outdoor archetypes at three levels of detail:

  • A generic level 1 covers situations in which emission location or conditions are unknown, and reflects a population-weighted average intake fraction of 39 ppm across 3,646 cities. To provide finer levels of detail, additional aspects to discriminate intake fractions from outdoor sources are needed, such as different air exchange rates and occupancy levels for indoor environments, city size, spatially differentiated meteorological conditions (dilution rates defined from mixing height and wind speed), and population distribution in relation to emission source distribution for outdoor environments.

  • Level 2 represents urban areas at the level of continental and subcontinental regions to ensure consistency between population, area and exposure.

  • If emission scenario information is available for specific cities, level 3 archetypes reflect PM2.5 fate and exposure conditions as precisely as possible, building on available intraurban outdoor intake fractions for 3,646 global cities parametrized for city-specific population, area, dilution rate, and PM2.5 background concentration, and combining these with population, area, and wind speed, based on high-resolution spatial data for rural environments.

Indoor exposure is highly dependent on air exchange and available volume per person. Because detailed building-specific air exchange and occupancy are usually not available at the requisite level of detail, the archetypes have to be defined to capture heterogeneity in indoor environments for providing reliable indoor intake fraction estimates. As with the outdoor archetypes, the researchers distinguish indoor archetypes at three levels of detail:

  • A generic level 1 is defined when emission location and building characteristics are unknown, reflecting average exposure conditions under residential indoor settings.

  • At intermediate level 2, intake fractions are discriminated according to different air exchange rates, occupancies, recirculation rates, and filter efficiencies for residential indoor settings and according to different ventilation rates and occupant densities for occupational indoor settings.

  • If emission scenarios are available for individual building types, intake fraction estimates at level 3 can be derived from specific air exchange, occupancy, and recirculation/filtration characteristics along with defining the building’s specific city or rural area.

Among their findings:

  • Across 3,646 urban areas with more than 100,000 inhabitants, the mean effective population-weighted intake fraction for urban ground-level emissions is 39 ppm (95% confidence interval: 4.3−160 ppm, median x̃ = 26 ppm). The full range of effective intake fractions across urban source environments spans from 0.9 to 280 ppm with a squared geometric standard deviation (GSD2) of 4.7.

  • Population-weighted effective intake fractions across urban areas per region vary from ∼10 ppm in Northern regions and Oceania to 57 ppm in Southeast Asia, with India as high-end subcontinental region at 70 ppm. This distribution corresponds well with the distribution of effective intake fractions in rural ground-level source environments showing a global mean population-weighted intake fraction of 2.2 ppm, ranging from 0.02 in Northern regions with tight buildings (low air exchange) and low occupancy to 4.2 ppm in Southeast Asia with typically high air exchange and high occupancy.

  • Even for outdoor emissions, between 83% and 90% of the intake takes place indoors, reflecting the high fraction of the day spent indoors.

  • Across indoor source environment archetypes, the mean effective intake fraction is 0.013 (13,200 ppm) for residential settings and 0.017 (17,200 ppm) for occupational settings, when the distribution of residential and occupational spaces in the different regions has not been considered.

  • For buildings without recirculation/filtration, effective intake fractions range over 3 orders of magnitude from 470 ppm in regions where buildings have high air exchange and low occupancy to 62,200 ppm in regions where buildings have low air exchange and high occupancy.

  • Indoor exposure contributes 91−99% to effective intake fractions across indoor source environments and is highest for conditions with high occupancy, low air exchange, and recirculation/filtration of indoor air.

our source-to-exposure framework provides for the first time a modular, fully mass balanced and flexible approach to combine PM2.5 exposure indoors and outdoors from emissions to residential or occupational indoor, and urban- and rural-outdoor environments. This approach provides a sound basis for integrating PM2.5 exposure assessment with multimedia models used to account for other substances potentially contributing to human disease burden. The main output of our framework is a set of effective indoor-outdoor population intake fractions reflecting three levels of detail based on a set of archetypes for different source environments.

… Results from applying our framework highlight that indoor exposure is an important contributor to overall exposure from PM2.5 emissions outdoors and that our set of archetypes can much better represent the variability between urban and rural outdoor exposure than equally or even more data-intensive spatially detailed models and moreover allows us to consider indoor environments.

—Fantke et al.


  • Peter Fantke, Olivier Jolliet, Joshua S. Apte, Natasha Hodas, John Evans, Charles J. Weschler, Katerina S. Stylianou, Matti Jantunen, and Thomas E. McKone (2017) “Characterizing Aggregated Exposure to Primary Particulate Matter: Recommended Intake Fractions for Indoor and Outdoor Sources” Environmental Science & Technology doi: 10.1021/acs.est.7b02589



If we keep burning wood, coal, oil and NG, at the current increasing rate, many species (including humans) will have a difficult time to survive in good health.

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