Although emissions from coal-fired power plants and their atmospheric reaction products contribute to environmental air pollution and are often cited as among the important sources of pollution-related health risks, there has been surprisingly little toxicological evaluation of the health hazards of breathing coal emissions or of the influences of coal type, operating variables, emission reduction strategies, and atmospheric reactions.  Virtually no toxicological research has been done to place “downwind” (i.e., rather than “top of stack”) emissions into context regarding population health risks.

The National Environmental Respiratory Center (NERC) evaluated the feasibility of generating exposure atmospheres simulating key components of human exposures to emissions from coal-fired power plants.  The goal of the NERC effort was to develop a plan for generating meaningful, reproducible, repeated inhalation exposures of sufficient numbers of animals for a sufficient time to characterize health hazards, and the exposure variables modifying those hazards. Important elements of this evaluation included a workshop of technical experts in this field, followed by a review by NERC's External Scientific Advisory Committee (ESAC).  It was concluded that useful exposures could be generated within the bounds of plausible technology and funding.

The workshop of technical experts, held in February 2003, produced a consensus of general specifications for the key components of a “downwind” exposure model, including:

• Sulfate particulate matter (PM) to fly ash PM ratio is ~ 100:1.
• Carbon content of fly ash is ~ 5–10%.
• Sulfur dioxide (SO₂):sulfate (SO₄) molar ratio is ~ 1:1.
• Total sulfur to nitrogen species molar ratio is ~ 2:1.
• Among nitrogen species, we want ~ 20% nitric oxide (NO), 55% nitrogen dioxide (NO₂), 10% peroxyacetyl nitrate (PAN), and 15% nitric acid (HNO₃).
• It is not worth attempting to fully model ozone or other secondary reaction products resulting in part from coal emissions.
• The upper bound of particle size should be limited to the respirable range for the species to be exposed.

It was determined that a suitable exposure atmosphere might be developed beginning either by combusting coal or by resuspending ash collected from a power plant, although neither approach would provide the desired atmosphere without modification by adding components and/or aging and reacting the emissions.  Although opinion among the technical experts was divided regarding the preferred approach, the preference of the ESAC was to begin by combusting coal.  It was determined that the most feasible laboratory-scale combustion apparatus for this purpose was the “drop-tube” furnace.