PM2.5 Exposure Calculator

Inhalation dose converts an ambient concentration into an absorbed mass by multiplying concentration by ventilation rate, exposure duration, and a deposition fraction. It is a more informative exposure metric than concentration alone, because a cyclist breathing 1.5 cubic meters per hour at 80 micro-g/m3 for one hour can take in more particulate than an office worker at 35 micro-g/m3 across a full shift. Ventilation rates from the EPA Exposure Factors Handbook run roughly 0.4 to 0.5 cubic meters per hour at rest, about 0.8 to 1.0 for light activity, and 1.5 to 3.0 for heavy exertion; the per-kilogram figure uses the conventional 70 kilogram reference adult. The deposition fraction is well below one because much of what is inhaled is exhaled again. For the accumulation-mode particles that dominate PM2.5 mass, total respiratory deposition is typically 30 to 60 percent, with a minimum near 0.3 micrometers where neither inertial impaction nor Brownian diffusion is efficient, so entries above 0.8 are optimistic. Useful reference concentrations are the World Health Organization 2021 guidelines of 5 micro-g/m3 annual and 15 micro-g/m3 over 24 hours, against US standards of 9.0 and 35 micro-g/m3. Epidemiological risk functions are fitted to ambient concentration rather than internal dose, so this figure supports exposure comparison, not direct risk prediction.

Concern with fine particles specifically, rather than total suspended particulate, followed the deadly smog episodes of the twentieth century, notably Donora, Pennsylvania in 1948 and the London smog of December 1952, which was later associated with roughly 12,000 excess deaths. Size-selective sampling developed through the 1970s as cascade impactors and dichotomous samplers showed that the fraction below about 2.5 micrometers penetrates to the alveolar region while coarser material deposits in the upper airways. The epidemiological case was made by two American cohort studies: the Harvard Six Cities Study, reported by Douglas Dockery and colleagues in the New England Journal of Medicine in 1993, and the American Cancer Society cohort analysis led by C. Arden Pope in 1995, both finding mortality gradients tied to fine particle concentration. The EPA established the first PM2.5 standards in 1997, revised the 24-hour value to 35 micro-g/m3 in 2006, and lowered the annual standard to 9.0 micro-g/m3 in February 2024. The dosimetry side, including deposition fractions by particle size and breathing pattern, comes from the International Commission on Radiological Protection human respiratory tract model published as ICRP Publication 66 in 1994.