• hazard, proprietary or "trade-secret" concerns may block access to information that is needed for prompt and appropriate remedial action.
• Finally, as emphasized in the first report, many exposed populations are so small, the period of observation so short, the exposure so poorly measured, or the outcomes so poorly defined that a verdict of "not proved" is all that can be attained. The situation is usually poorly suited for the conduct of research regardless of the efforts that are expended and the skills of the investigators.

Nevertheless, given the great and increasing variety of chemical, physical, and biologic pollution in the environment, the first indication of a hazard from a particular chemical or group of chemicals may still follow an investigation of some event, or cluster, by a public-health department or concerned citizens. Reports from state and local public-health groups may then enter the gray literature and in some cases be the starting point for research that is published in peer-reviewed journals. These locally initiated studies, although using small populations with poorly characterized exposures, may suggest an effect. Efforts to develop databases of such studies might serve 2 purposes: other parties interested in similar exposures could learn what studies are in progress, so as to increase sharing of information on study design; and appropriate analysis of the combined study findings might eventually become feasible, as discussed further in chapter 7.

The focus of this report is on environmental-health issues in the developed world, but it is recognized that in developing countries environment-related diseases occur along with the more predominant infectious and chronic diseases. Severe indoor particulate air pollution has been documented in the developing world at levels 100 times higher than the US standard of 150 µg/m³ for particles less than 10 µm in diameter (PM,₁₀), and outdoor pollution is also sometimes extreme. These high levels of air pollution, coupled with other disadvantages in the developing world, may account for the fact that acute respiratory disease is the second leading cause of death in children under 5 years of age in countries of the developing world (Leowski, 1986). Important risk factors for both infectious and chronic diseases include basic sanitation, living conditions and urban infrastructure, housing, air and water pollution, and working conditions. All of these are threatened by the sanitary burden that is being accumulated in developing countries. These are the areas in which hygienists have had a great impact in the past in developed countries.

For many chronic, degenerative diseases of potential interest in environmental epidemiology, data on rates of occurrence (incidence) in de-