Polluting your internal environment: homeostasis is a factor in health. (Science Selections).

Numerous studies conducted around the world have shown a correlation between air pollution and adverse health effects, including premature death. But researchers have been stumped when they try to pin down a specific, consistent mechanism by which different air pollution mixtures affect people with varying susceptibilities living in a range of climates and settings. Researchers at the Johns Hopkins Bloomberg School of Public Health propose an experimental model to test whether a key factor in vulnerability to air pollution--related health effects might be the body's homeostatic capacity, or ability to compensate internally for external stressors and maintain a stable internal environment [EHP 110:61-65]. In particular, they are interested in changes in daily death rates.

The concept of homeostatic capacity has been around for decades, but there are few hard data on which indicators of homeostatic capacity are most important, what indicator levels portend a body's impending collapse, or what effects that outside stressors, such as pollutants or pronounced temperature swings, might have on individuals with reduced homeostatic capacity. It is known, however, that homeostatic capacity declines with age and illness, and that as this decline proceeds, the risk of death increases; the number of stressors capable of causing death goes up, and an individual stressor need not be as severe or intense to cause death.

The Johns Hopkins team evaluated a small number of short-lived inbred mice for two indicators of homeostatic capacity: deep-body temperature and electrocardiographic heart rate. After surgically implanting a radiotelemeter that continuously measured the two indicators, researchers monitored the daily routine of the animals.

Deep-body temperature in the mice fell progressively during the weeks before natural death. In one mouse described by the authors, deep-body temperature fell about 9 [degrees] C over the last 11 weeks of life, and previously predictable temperature oscillations from day to night became erratic. In about the same time frame, heart rate slowed significantly.

Because such significant declines correlated closely with the demise of the animals, they may prove to be good markers of impending health problems in humans as well, says Robert Frank, one of the team leaders. Temperature and heart rate might be particularly good markers for humans because they frequently are measured noninvasively during routine medical examinations and are associated with many key body processes. Other homeostatic indicators that could be monitored include body weight, blood oxygen concentrations, blood viscosity, and extra- and intracellular pH and salinity.

The authors say that regardless of the indicator selected, limited homeostatic capacity in infants and young children, the elderly, and people suffering from a wide range of diseases may be a key element in making them more vulnerable to a wide range of environmental stressors, including not only air pollutants but many other stressors as well. "You're dealing with a population that's gotten to the point where any nonspecific threat can be the proximate cause of death," Frank says. "These are people for whom there is very little benefit to eliminating a single proximate cause of death because there is a whole universe of such causes."

Based on existing data, eliminating a few select air pollutant stressors would most likely be effective in reducing only acute health problems, which might protect less than one-tenth of 1% of the general population, according to the authors. The researchers' hypothesis and the model they are developing to test the hypothesis were intended to explain premature loss of life measured in days or weeks. They suggest, however, that the same model might be used to test whether chronic exposure to polluted air is associated with subtle long-term shifts in homeostatic capacity, which could contribute to chronic health problems and to greater shortening of the life span.