Whose Particle Is It? Source Apportionment and the PM2.5 Background Problem
By Paul E. Rosenfeld, M.S., Ph.D., Principal Environmental Chemist, RoseyFields LLC, Santa Monica, California
The hardest question in particulate matter litigation
Fine particulate matter (PM2.5) rarely comes from a single identifiable source; it arrives from industrial combustion, vehicle traffic, residential heating, wildfire smoke, and secondary atmospheric chemistry all mixed together in the same air a plaintiff breathes. RoseyFields LLC’s air dispersion and exposure assessment practice addresses PM2.5 litigation specifically through source apportionment methodology designed to isolate a defendant’s incremental contribution from that mixed background.
The current federal health basis for concern
EPA’s health assessment materials for PM2.5 describe associations with cardiovascular outcomes, including heart attack, stroke, and cardiovascular mortality, and respiratory effects including asthma exacerbation, based on an extensive body of epidemiological and clinical evidence spanning several decades (US EPA, PM2.5 Health and Environmental Effects).
A regulatory standard that has tightened over time
EPA’s current primary annual PM2.5 National Ambient Air Quality Standard is 9.0 micrograms per cubic meter, reflecting a series of periodic reductions from earlier, less protective standards as accumulating epidemiological evidence demonstrated health effects at progressively lower concentrations (US EPA, National Ambient Air Quality Standards).
The foundational cohort studies behind the current standard
The Harvard Six Cities Study established a landmark association between long-term particulate exposure and mortality across six U.S. cities with differing pollution levels, and the subsequent American Cancer Society cohort study extended that finding to lung cancer and cardiopulmonary mortality across a much larger national population, together forming the epidemiological backbone for PM2.5 regulation (Dockery et al., 1993; Pope et al., 2002).
Why a with-source and without-source model is the core analytical tool
Rather than attempting to prove that a defendant’s facility created all of the PM2.5 a plaintiff was exposed to, a defensible source apportionment analysis models total ambient concentration with the defendant’s emissions included and then models the same period with those emissions removed, isolating the specific incremental concentration increase attributable to that single source.
Chemical fingerprinting as a complementary line of evidence
Beyond dispersion modeling, specific chemical constituents and elemental or carbonaceous markers in collected particulate samples can sometimes help distinguish an industrial combustion source’s contribution from traffic-related or regional background particulate matter, though the specific markers available depend heavily on the facility type and analytical data collected.
Secondary particulate formation complicates simple stack-to-receptor modeling
Some PM2.5 forms not at the emission point but through atmospheric chemical reactions of precursor gases, meaning a facility’s contribution to community PM2.5 exposure is not limited to particles physically emitted from its stacks; a complete source apportionment analysis must therefore account for precursor gas emissions and their downwind atmospheric transformation, not only directly emitted particulate matter.
Historical reconstruction for facilities with changing operations
As with other industrial exposure litigation, a facility’s production rate, fuel type, and pollution control equipment can change substantially over the years relevant to a historical exposure claim, meaning today’s emissions profile cannot simply be applied backward to reconstruct exposure during an earlier operating period.
Why regulatory compliance and litigation causation remain separate questions
A facility’s compliance with the current PM2.5 NAAQS reflects a population-level regulatory determination and does not, by itself, answer whether that facility’s specific historical emissions reached a specific plaintiff’s residence at a specific concentration during a specific exposure period, just as an estimated exceedance of a health-based benchmark does not by itself establish that a specific plaintiff’s disease was caused by that exposure. Dr. Rosenfeld discusses his particulate matter source apportionment methodology further at RoseyFields.com.
References
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12. Rosenfeld PE. RoseyFields LLC: Particulate Matter Source Apportionment and Air Dispersion Modeling. Santa Monica, CA: RoseyFields LLC; 2026.
13. US Environmental Protection Agency. National Ambient Air Quality Standards Table. Washington, DC: US EPA.
14. Health Effects Institute. State of Global Air. Boston, MA: HEI; 2024.