Introduction

Concern for the adverse health impacts of air pollution continues to be the principal (although not sole) motivation for air quality and source emissions regulations.  Accordingly, the need to better serve research on air quality-health relationships is a major justification for improving the speciation of organic environmental air contaminants. 

Many air contaminants of health and regulatory concern are wholly or in part organic carbon.  For example, among the six National Ambient Air Quality Standards (NAAQS) pollutants there is considerable current concern for the health impacts of ambient particulate matter (PM) (EPA, 2003).  A small fraction of coarse PM mass (PM_10-2.5) consists of organic material of animal or plant origin, or is contaminated by natural (e.g., bacterial endotoxin, plant pollen protein) or anthropogenic (e.g., condensed combustion products) organic carbon.  However, organic carbon, largely anthropogenic, typically comprises 20-40% of fine PM (PM_2.5).  Within PM_2.5, there is considerable interest in, but little knowledge about, the health implications of the “ultrafine” (nominally £100 nm) and “nanoparticle” (nominally £ 50 nm) fractions, which although comprising a minor portion of total PM mass, dominate PM counts.  These smallest fractions of ambient PM contain greater portions of organic carbon than PM_2.5; indeed, many nanoparticles consist almost entirely of condensed semi-volatile organic compounds (SVOCs), or non-volatile organics.  Beyond the NAAQS, EPA identified 188 Hazardous Air Pollutants (HAPs) considered to present important health hazards <http://www.epa.gov/ttn/atw/pollsour.html>, the majority of which are organic carbon compounds or classes.  Among the HAPs, EPA has identified 33 Urban Air Toxics (UATs) of greatest concern 25 <http://www.epa.gov/ttn/atw/urban/list33.html>, 25 of which are organic carbon compounds or classes.  

People always breathe complex mixtures of many air contaminants from many natural and anthropogenic sources.  There is growing recognition of the need for a better understanding of the contributions of the many different individual air contaminants, and classes of contaminants, to the health effects associated statistically with air pollution.  Although some efforts have been directed toward “dissecting” the chemical species responsible for the biological effects of certain source emissions, most ambient air quality health research has focused on individual pollutants.  This approach has fostered considerable improvement in air quality to date, but the relationship between air quality and health is unavoidably a multi-pollutant, multi-source issue.  To better understand the relationship, we need to answer several interrelated questions.  Which air contaminants are causally related to which health effects?  Which are the most important?  To what extent can we lump pollutants and pollutant classes for regulatory purposes?  What are the contributions and relative importance of different pollution sources?  Are there combinations of air contaminants for which we should be concerned (i.e., that have health impacts different from those predicted on the basis of the single components)?  What plausible changes in air quality would yield the greatest, most cost-effective benefit?  A fundamental question underlies these issues: what is the relationship between the composition of complex pollutant mixtures and their health hazards and risks?

Moving beyond our present understanding of the air quality-health relationship will require a better understanding of exposure which, among other needs, will require more detailed information on the physical-chemical species of organic carbon compounds to which people are exposed.

Findings

Answers to the Questions

1)      Is there good evidence for the health importance of organic air contaminants?

Yes!

The NAAQS for PM_2.5, the list of HAPs, and the subgroup of UATs would not exist if there were not substantive evidence that at some level of exposure, these air contaminants are harmful to health.  There is a huge literature demonstrating that individual organic compounds and classes that are present as air contaminants are harmful to health.  No attempt is made here to review that literature, but one has only to access the information sources cited in the introduction to gain an appreciation for its extent.

There is information from human and/or animal studies that the individual HAPs, and many organic species not included in the HAPs, can, at some dose, cause a wide spectrum of adverse health effects including irritation, inflammation, non-cancer diseases of multiple organs, cancer (or effects leading to cancer), birth defects, neurological abnormalities, and enhancement of allergic responses.  A few recent examples will illustrate the spectrum of evidence.  An epidemiology study demonstrated associations between cardiovascular emergency department visits in Atlanta, GA and PM-borne elemental carbon, organic carbon, and oxygenated hydrocarbons, among other pollutants (Metzger et al., 2004).  Using filtered and unfiltered diesel emissions, it was shown that gases, vapors, and nanoparticles were more responsible than “soot” for lung inflammation in experimentally exposed humans (Rudell et al., 1999).  Personal exposure to PM_2.5 and polycyclic aromatic hydrocarbons (PAHs) were found to correlate with the level of DNA adducts (abnormal attachments of organic moieties to DNA) in both a general population and coke oven workers in the Czech Republic (Lewtas, et al., 1997).  The organic components of combustion PM (including SVOC species) were found to amplify human nasal allergic responses (Nel et al., 2001).  PM and vapor-phase SVOC fractions of emissions from different vehicles was found to vary considerably in toxicity in animal lungs (Seagrave et al., 2002), and statistical analysis of composition-response relationships revealed that hopanes and stearanes, markers of engine oil, co-varied most closely with toxicity (Mauderly et al., 2003).  Detailed analysis of the composition of petroleum samples allowed differences in mutagencicity in bacteria to be associated with certain chemical “fingerprints” (Eide et al., 2002).

2)      How is our current knowledge of the air quality-health relationship limited by the present lack of analytical data?

You can’t study what you don’t measure!

Most research funding is focused on single pollutants or sources involved in regulatory debates, and the majority of debate is driven by reviews of the NAAQS.  Among the NAAQS, only the PM standard involves organic carbon.  Although research advisors have noted the need to better understand the contribution of organic carbon to PM effects (NRC, 2003), efforts to date have not been widespread.  A major factor is that epidemiology has largely relied on “exposure” (ambient air quality) data collected for regulatory compliance, and there is no current requirement to measure anything other than PM mass concentrations.  (HAPs are “controlled” on the basis of source emission factors, not on the basis of measured ambient concentrations.)  The complexity and cost of sampling and analysis for detailed organic speciation hinders its deployment in prospective epidemiology.  Similarly, although laboratory health studies are prospective in nature, conducted in a single research environment, and could take advantage of contemporary organic speciation methods, few studies have done so.  A limited number of studies have incorporated analyses of exposures to organic carbon at various levels of detail, but overall, the lack of exposure data has severely limited research on the health impacts of organic carbon. 

Health research is also limited by the sparse characterization of ambient airborne organic matter aside from “health” studies.  For example, a better understanding of the spatial homogeneity of classes of organic matter would allow epidemiologists to more efficiently incorporate organic analyses into prospective studies.  It is presently difficult to judge the number and location of samplers necessary to address exposure-effect hypotheses.  Similarly a better understanding of the co-variance of concentrations of different classes of organic compounds among locations would facilitate study design.  Many classes of organic and inorganic pollutants co-vary because they are derived from the same sources and/or are affected similarly by meteorology.  Knowledge of locations where, and the extent to which, different classes of organics might be “unlinked” would greatly facilitate the targeting of environmental studies to resolve exposure-effect relationships.

Although even the current state-of-the-science limits the extent to which complex organic composition can be resolved, the principal problem of past research has not been an inability to conduct analyses (i.e., analytical capability).  The principal problem has been a failure to incorporate analyses into research programs and individual experimental designs (i.e., analytical availability).  

3)      How could health researchers utilize improved information?

In many ways!  The problem is not a lack of plausible research strategies, it’s primarily a lack of incorporation of organic speciation measurements into health research protocols.

The full range of demonstrated and potential experimental designs will not be discussed in detail here, but the following examples illustrate a spectrum of research strategies in which health researchers could make use of improved organic speciation.  Improvements are needed in both the amount of data that are generated, and the detail with which organic composition is resolved.

One general strategy is to test statistical associations between exposures of humans to organic species and adverse health outcomes.  Epidemiological studies test statistical relationships between exposures and health outcomes among either small, select populations (“panel studies”) or larger area populations (“ecological studies”).  The degree to which such studies can detect effects of organic compounds depends on the accuracy with which the exposures are actually known.  Samplers attached to the individual or in the individuals’ specific locations can provide specific exposure information in panel studies.  Larger area samples are typically used in ecological studies.  An example is the recent finding of an association between cardiovascular emergency department visits and concentrations of organic components of PM in the Atlanta, GA area (Metzger et al., 2004).  The Aerosol Research and Inhalation Epidemiology Study (ARIES) is among the few attempting to include classes of organic carbon in city-scale epidemiology (Van Loy et al., 2000).  In some cases, biological markers of exposure to organic species can be used to better test links between exposures and health outcomes (Talaska et al., 1996).  For example, Lewtas et al. (1996) linked increased concentrations of DNA adducts in blood cells of general and worker populations to levels of exposure to PAHs.  The use of chemical analysis for source apportionment can also contribute to the interpretation of population studies (e.g., Maykut et al., 2003). 

A second general strategy is to intentionally expose biological systems (e.g., humans, animals, or cultured bacteria or mammalian cells) to specific organic compounds and examine specific biological responses.  This strategy can involve administration of either selected compounds or complex mixtures.  Nels et al. (2001) for example, used complex organic extracts to determine that the organic fraction of diesel soot plays a role in amplifying human nasal immune and inflammatory responses to allergens, and then used a single compound to determine that pyrene, a “model” PM-associated PAH, can also cause the effect.  This general approach is the one most widely used to test the toxicology of compounds in animals, and has provided much of the data supporting selection of the HAPs. 

A third general strategy uses “chemical dissection” to determine the compounds or classes primarily within complex mixtures that drive a health response.  The best-known example is the “biodirected fractionation” of solvent extracts of combustion PM, in which progressive fractionation and testing of organic matter proved that nitroaromatic compounds are largely responsible for mutagenic activity in bacteria (Scheutzle and Lewtas, 1986).  Testing the roles of organic extracts vs. black carbon in the nasal immune response to diesel soot (Nels et. al, 2001), and testing the effects of filtered and unfiltered engine emissions on lung inflammation (Rudel et al., 1999), are variants of this approach.  Current analytical technology is usually adequate for studies of single known compounds.  One example of a potential inadequacy is the lack of information on the health importance of the “unresolved complex mixture” that is not currently collected or speciated in analyzing complex organic mixtures.  It would be useful to isolate quantities of unresolved material sufficient for toxicity screening in simple biological systems.  If the material is shown to have biological activity, resolving its composition and sources would be important.

A fourth general strategy involves the use of “statistical dissection” to determine the components of complex mixtures causing health responses.   If composition and responses are measured identically for all multiple complex exposures having different but overlapping compositions, the results can be combined into a composition-response data matrix and analyzed statistically to determine the physical-chemical species that co-vary most closely with differences in response.  An example of this approach is the use of Principal Component Analysis and Partial Least Squares Regression (PCA/PLS) to determine the compounds in petroleum samples contributing most strongly to mutagenicity in bacteria (Eide et al., 2002).  Another recent example is the use of PCA/PLS to determine that engine oil components contributed most strongly to differences in lung toxicity among a set of engine emission samples (Mauderly et. al., 2003). 

Overall, there is ample opportunity for a greater incorporation of organic analyses into health research.  In some cases, adequate analytical technology exists, and the issue is directing greater attention to health research on organic components of air pollution.  In some cases, analytical instrumentation exist as research tools, but are not evolved into packages sufficiently standardized, simple, and inexpensive for widespread deployment.  In a few cases, current analytical capabilities may not be sufficient even as research tools.

4)      How can interactions between the analytical and health research communities be improved?

We need improvements in both communication and technology!

There needs to be more dialogue between health researchers and the analytical community.  A substantial portion of the health research community are insufficiently knowledgeable about the actual composition of air pollution (especially the organic components), how pollution “works” (source emissions, transport, and atmospheric chemistry), and analytical possibilities (what can be measured and how) to conceive innovative experimental designs.  Many among the analytical community may not be sufficiently knowledgeable about the fundamentals (not the details) of the nature and range of health effects, the physical-chemical interface between airborne organic species and biological fluids and tissues, likely cellular-molecular biological mechanisms of effects, and plausible biological experimental approaches (and their inherent limitations) to perceive the range of application of their technology to health research.  Not only do we need more tutorial and “brainstorming” dialogue, there also needs to be a larger number of “bridging” scientists who consider themselves part of both research communities.  

We need to take better advantage of several pathways for cross-fertilization between analysts and biologists.  The publication by each community of its papers in its own journals is necessary and valuable, but is not an effective pathway for cross-disciplinary communication.  There needs to be more development and dissemination of summary, tutorial, publications.  The EPA Criteria Documents (e.g., EPA, 2003) contain the needed information for NAAQS pollutants, but few researchers read sections pertaining to other disciplines.  The recent publication by NARSTO, “Particulate Matter Science for Policy Makers” (2003) is an excellent example of a tutorial synopsis that would be useful to biologists, although the title may not suggest its value to the health research community.  Developing appropriate materials is only the first step; without distribution across disciplines, communication does not occur.  Presenting both air quality and health sessions at scientific meetings in another pathway; however, the common practice of doing so in parallel sessions often hinders real cross-education.  The conduct of cross-disciplinary tutorial sessions at scientific meetings (unopposed, when possible) is an excellent strategy, but not frequently implemented.  “Center” programs containing both analytical and biological components (e.g., NIEHS Environmental Centers and EPA PM Centers) can be very productive, but typically struggle to truly integrate the disciplines into unified research strategies. 

There needs to be more emphasis on training researchers that span scientific disciplines, in order to expand the number and types of scientists conceiving and conducting truly integrative research.  The NIEHS Mentored Quantitative Research Career Development program is an example of such an effort (http://grants1.nih.gov/grants/guide/pa-files/PA-99-087.html).  Federal agencies, states, and professional organizations could sponsor more training grants aimed specifically at cross-disciplinary training.

Finally, there needs to be greater attention among funding organizations to incorporating organic analytical capabilities into health research.  Simply getting the two communities to communicate is worthwhile, but does not necessarily result in joint research efforts.  Scientists are most strongly motivated to action by the structure of funding opportunities.  If research solicitations are framed to merge the two communities into joint research strategies, the scientists will conceive creative ways of doing so.  Conversely, if research solicitations do not place a premium on such interactions, they are much less likely to occur. 

Although not the key topic of this workshop, it should be noted that a need for better cross-fertilization pertains to links within the health and analytical communities, as well as between those communities.  It is clear that more strategic interactions and coordination is needed among different health disciplines and among researchers focused on different health outcomes.  It is likely that the same might be true within the analytical community.

We also need technological advances that: 1) make a greater level of characterization of exposures to organic species practical for more widespread deployment in field and laboratory studies; and 2) provide a more thorough speciation of complex organic mixtures.  At present, the former is more critical than the latter.  We know enough to be confident that applying the current level of speciation capability more widely in health research could result in tremendous advances in our understanding of the health importance of airborne organic carbon (in all physical phases) – even though we don’t know what the answer might be.  We need to make analytical techniques and equipment more practical for wide deployment, including use by non-specialists (simpler, faster, cheaper; that’s right – all three!).  It is also possible to apply advanced analytical technologies at the prototype or “research” stage in limited health studies.  For example, it would be useful to know whether or not the major fraction of complex organic matter that cannot now be readily resolved might have health importance (or conversely, whether we can relax and ignore it).  Exploratory work using prototype technologies can be done with small samples and simple biological systems (e.g., cultured cells) or limited complex biological systems (i.e., instilled into lungs of small numbers of animals). 

Conclusions and Recommendations

There is a serious need for research providing a better understanding of the health impacts of the many different air contaminants that comprise exposures to air pollution, and many of those contaminants are organic carbon compounds.  That research will require various levels of speciation in order to test associations between health effects and different classes and compounds.  Both improved dialogue between the health and analytical research communities and advances in analytical technology are needed.  The key communications challenges presented to the analytical community by health research needs are to take an active role in 1) educating the health research community about exposures to organic compounds; and 2) facilitating incorporation of organic analyses into health research.  The key technological challenges are to: 1) expand the range of analytical instrumentation that is sufficiently transportable and inexpensive to deploy widely in health research; and 2) to develop methods to resolve organic carbon composition more completely.

References

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