Presentation Abstract

Ambient Particles, Their Toxic Components and Sources, and How They Impact Health
John R. Froines
Southern California Particle Center and Supersite, Los Angeles, CA

During the first five years, the Particle (PM) Centers have developed important information on how particle characteristics, including size and composition, vary with source, season and atmospheric processes in relation to toxicity and health outcomes. This presentation will review key ambient PM studies and highlight selected results. We will review studies examining PM sources, including atmospheric chemistry, review freeway/mobile source studies, and highlight the importance of studying mechanistic hypotheses to elucidate the relationship between PM characteristics and toxicity.

Research in the Los Angeles Basin has been geared towards exploring the differences in PM as it relates to sources. Three zones of influence have been studied: freeways, urban source areas, and sites downwind of emission sources (receptor sites). Atmospheric chemistry is an important source of toxic PM components at receptor sites. Data collected on PAHs show that as PM moves across the Los Angeles basin from west to east, the size modes of the PAHs shift from a predominance of PAHs in the ultrafine mode at the source, to PAHs in the accumulation mode at the receptor. Studies have demonstrated that highly toxic organic molecules capable of redox chemistry are formed via atmospheric chemistry thereby resulting in increases in airborne concentration at receptor sites. By doing studies directly adjacent to a freeway, the immediate zone of influence of mobile sources has also been explored in terms of distance from the freeway and season. Work is also being conducted on determining “tracers” for source emissions and using those tracers as a means of studying emissions, toxicity and resulting health effects.

PM Centers’ research has established that in addition to respiratory effects of PM, neurological, cardiovascular, and developmental endpoints are of concern. Many studies have been conducted in mice, rats and humans to explore the effects of freeway exposures on health endpoints. Cardiovascular effects studies, conducted with both aged rats and healthy humans, showed changes in heart rate variability and blood pressure. Inflammatory responses from PM exposure in close proximity to freeways have been demonstrated. The Children’s Health Study showed that children living by a freeway were more likely to develop asthma and two recent New England Journal of Medicine papers have demonstrated cardiovascular risk from traffic exposure and clinically significant impaired lung growth in children. Being close to freeways during pregnancy also has been shown to cause low birth weight and pre-term births, particularly for winter births when exposure is higher.

The mechanistic understanding of how PM affects health has improved considerably. It has been demonstrated that PM contains redox active chemicals capable of producing oxidative stress. Reactive oxygen species (ROS) cause induction of antioxidant defenses, inflammation and cell death. It has also been shown that ultrafine particles, on a per mass basis, are the most potent in terms of the generation of oxidative stress. Ultrafine PM from mobile sources has been shown to be preferentially taken up by mitochondria leading to the disruption of the organelle and cell death.

Important issues for future work include: Which sources pose the greatest risk to public health? What are the most critical characteristics of PM in relation to toxicity? Which health effects are most sensitive to low levels of PM exposure?

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