Debate continues about what constitutes significant and meaningful change in health status of individuals and populations. More importantly, the basic biological and medical criteria that are used for clinical and environmental judgments require further discussion and clarification. What proportion of loss of cardio-pulmonary function, overt disability, or mortality is sufficient to determine an "adverse health effect"? Health-oriented individuals, including researchers and clinicians, may choose to adhere to different criteria than other professional groups (e.g., legal, social). It is proposed in this paper that criteria for defining adverse health effects should represent clinically meaningful, as distinct from only statistically significant, responses. These include pulmonary function test results that indicate obstructive or restrictive diseases, and electrocardiogram results indicating coronary artery disease. Intraindividual changes that predict a meaningful medical change would be included; these changes should meet specific requirements in terms of what constitute normal vs. abnormal ranges of variation. Further, the proportion of the population defined to be impaired should be considered. These issues are the focus of this paper.