in the US and around the world have provided
evidence that air pollutants, especially the small suspended particles whose diameters can be
hundreds of times less than the thickness of the human hair, can adversely impact pulmonary
and cardiovascular health. The US Environmental Protection Agency has used these findings as
the basis for establishing stringent national air quality standards for ambient particles.
In spite of the plethora of studies reporting associations between particle exposures and a spectrum
of morbidity outcomes and mortality, there are still many unresolved scientific issues. To address
these issues, Congress in 1997 appropriated funds to the EPA for the establishment of specific
academic Centers dedicated to conducting research. The result of this initiative was the establishment
of five university-based Centers located at the University of California at Los Angeles, Harvard
University, New York University, University of Rochester and the University of Washington. Each
of these groups has undertaken a variety of multidisciplinary projects which collectively will
generate the necessary scientific information needed by the EPA for developing air quality
standards that protect public health.
Given their minuscule size, inhaled ambient particles can readily deposit deep within the human lung.
Therefore, scientific investigations have historically focused on their respiratory system effects,
especially for individuals with respiratory diseases such as chronic bronchitis and asthma.
Recent studies, however, have shown that exposures to particles are also associated with cardiovascular
morbidity and mortality. Moreover, many scientists and the public in general, have been surprised
that relatively low concentrations of tiny particles can initiate a cascade of biological changes
that can adversely impact the function of the lungs, heart and vascular systems and, in some cases
may ultimately cause premature death. Research conducted at the five Centers aims to elucidate
the biological mechanisms whereby particles can induce such adverse effects.
Some of the research conducted by the Centers has provided evidence that particles that are deposited in the
lungs can be rapidly transported to the olfactory bulb of the brain and adjacent structures. This finding
may support one of the hypotheses that the central nervous system can potentially be an important target
organ, which may react by triggering changes in the function of the heart or other organs. Alternatively,
sympathetic or parasympathetic neuronal responses could be induced by inflammatory mediators released
upon the deposition of particles onto the respiratory system. Although Center researchers are still
in the processes of testing the plausibility of the different biological mechanisms, they have carried
out numerous experiments that have clarified the causal relationship between particles and adverse
human health. For example, a series of controlled exposure studies involving both humans and animals
using outdoor particles showed that exposures were linked to a number of biological responses,
including changes in the electric properties of the heart, arrhythmias, pulmonary inflammation
and asthma aggravation among others.
Another contribution that has emerged from the Centers has been the development of new statistical
methods that have made it possible to identify specific sub-populations at higher risk from particle
exposures. Epidemiological studies have shown that individuals with pre-existing cardiovascular
diseases are likely to be at greater risk from particles. Diabetics, for instance, who frequently
also suffer from vascular diseases have emerged in the Center studies as a previously-unidentified
susceptible population. In an effort to understand why individuals with certain diseases are at
greater risk than others, Center researchers have used genetically engineered animal models that
replicate respiratory, cardiac or vascular diseases such as chronic obstructive pulmonary diseases,
arteriosclerosis or diabetes. The majority of these toxicological studies supports the findings
of the epidemiological studies and, provides some insight why individuals with cardiac or vascular
diseases are at higher risk.
Since particles are a chemically and physically very diverse class of pollutants, a prime task of Center
researchers has been to identify specific components of PM that may be especially toxic. This research
has led scientists closer to determining whether some particle exposures may be worse than others.
This information is important for regulatory agencies, since controlling the right source, among
potentially hundreds of sources, is critical in our efforts to protect the public health in a
cost effective way. Center researchers have studied the toxicity of particles emitted from diesel
and gasoline-vehicles, power plants, wood stoves, re-suspended soil dust and laboratory-generated
model particles. To date, not all of these studies have been completed; however, research findings
highlight the importance of combustion sources. Particle-related mortality was found to be associated
with traffic and other combustion sources to higher degree than soil dust, for example. In addition,
in a study where mice were exposed to traffic particles at varying distances from a freeway, the
greatest inflammatory, neurological and immunological effects were observed for the group located
closest to the freeway. These results support findings from previous investigations showing increased
air pollution related risks for individuals living close to highways or heavily trafficked roads.
Considering the size of populations exposed to particles and the extent of associated health effects
such as premature mortality, medication use, hospital visits, school and work absentees among others,
one can conclude that particles pose a serious public health problem. For instance, particle-related
mortality in the US is estimated to be 64,000 deaths per annum (National Defense Research Counsel, 1996).
Therefore, pollution control strategies may be necessary to further reduce risks to an acceptable level.
Decisions on developing very costly public policies should always rely upon strong scientific evidence.
Researchers from the Centers and numerous institutions have already addressed many scientific issues;
however, as summarized by a recent National Academy of Sciences report, many uncertainties still exist
and thus research of particle-induced effects should continue.
last updated Jul, 31, 2006