How Atmospheric Coatings Increase PAHs' Global Reach and Health Risks
Imagine a tiny, soot-covered particle, smaller than a speck of dust, traveling thousands of miles through Earth's atmosphere. It originates from a factory in Asia, rides air currents across continents, and eventually finds its way into human lungs in North America.
Laboratory experiments showed benzo[a]pyrene (BaP) should break down within hours when exposed to ozone, yet field measurements detected it far from sources, indicating it persisted for days or weeks 3 .
A groundbreaking discovery revealed a protective mechanism: viscous organic coatings that shield these toxic molecules during their long-range transport 6 .
We've likely been underestimating the global lung cancer risk from these pollutants due to their extended atmospheric lifetime.
Polycyclic aromatic hydrocarbons (PAHs) are a large group of organic compounds composed of two or more fused benzene rings arranged in various structures 1 . They're formed through the incomplete combustion of organic matter—whether from fossil fuels, biomass, or even cooking 9 .
When we inhale PAH-containing particles, these compounds can initiate a complex process that may eventually lead to cancer. The parent PAH molecules themselves aren't the direct threat—they're what scientists call "procarcinogens" 1 .
The mystery of how PAHs persist in the atmosphere was solved when researchers discovered that these molecules become shielded from oxidation by coatings of viscous organic aerosol (OA) 3 .
Think of this protection like a chocolate-covered nut—the hard shell protects the nut inside from moisture and air.
These coatings vary dramatically with temperature and humidity. In cool, dry conditions, the organic coatings become viscous and semi-solid, creating an effective barrier 3 .
This temperature-dependent shielding creates striking geographical patterns in PAH persistence. The protective coating is more effective in middle and high latitudes compared to the tropics 3 .
Red markers show regions with high protective shielding
| Region Type | Coating Properties | PAH Degradation Rate | Long-Range Transport Potential |
|---|---|---|---|
| Cool/Dry (High Latitudes) | Viscous, semi-solid | Slow | High |
| Warm/Humid (Tropics) | Liquid-like | Fast | Low |
| Temperate Regions | Variable with season | Moderate | Seasonal variations |
The key insight about OA shielding emerged from sophisticated modeling studies that reconciled the discrepancy between laboratory observations and field measurements.
In 2017, Shrivastava and colleagues developed a novel modeling approach that incorporated the effects of temperature and humidity on organic aerosol viscosity and its protective capability 3 6 .
When the team applied this new approach, the results were striking: the revised model predicted higher atmospheric BaP concentrations that aligned much better with actual field measurements than previous models 6 .
Long-range transport across continents and oceans 3
Higher deposition fluxes in remote regions 3
Substantially elevated lung cancer risks 3
| Research Method | Application | Key Finding |
|---|---|---|
| Global Climate/Chemistry Models 3 | Simulating PAH transport | BaP lifetime extends from hours to days/weeks with shielding |
| Chemical Analysis 2 | Measuring PAH metabolites | Individual susceptibility affects lung cancer risk |
| Health Risk Assessment 7 | Estimating cancer risks | Without considering susceptibility, risk underestimated by 55% |
The discovery of the shielding effect has profound implications for how we assess the global health burden of air pollution.
3.1 × 10⁻⁵
Global risk from ambient PAH exposure 7 - approximately 3 additional lung cancer cases per 100,000 people exposed.
A startling 36.9% of the world's population lives in areas where ambient air BaP concentrations exceed the EU target value 7 .
Biomass fuels
Fossil fuel combustion
Coke production
Motor vehicles
| Environment Type | Typical BaP Concentration | Key Influencing Factors |
|---|---|---|
| Urban Air | 1-200 ng/m³ 1 | Vehicle traffic, industrial emissions, heating systems |
| Rural Air | Lower than urban but variable 5 | Agricultural burning, long-range transport from urban areas |
| Indoor Air | Highly variable | Cooking methods, heating fuels, tobacco smoke |
| Grilled Meats | Up to 69 μg/kg in high-PAH foods 1 | Cooking method, fat content, fuel type, cooking duration |
The discovery that organic aerosol coatings shield PAHs during atmospheric transport has fundamentally transformed our understanding of how these carcinogens move through our environment and affect human health.
What once seemed a contradiction between laboratory studies and field observations now represents a landmark example of how complex atmospheric processes can extend the reach and impact of pollution.
Scientists are working to better understand how climate change might affect the shielding process—as global temperatures rise, will the protective coatings become less effective?
Addressing the health risks from PAHs requires both global cooperation and local actions. By understanding the invisible shield that protects PAHs during their atmospheric journeys, we can develop better strategies to protect ourselves and future generations.