zone, small particles, mercury, lead. In Mexico, these are just a few of the many targeted items on the government's environmental health to-do list. Not surprisingly, getting that list down to size can be a tall order. "In countries like Mexico, which are characterized by huge inequalities and rapid economic and epidemiological transitions, the environmental health agenda is complex," says Dr. Julio Frenk, Mexico's new health minister and a former visiting professor to the Harvard School of Public Health. "Just as in many other areas of public health, we have a double burden to contend with. Without having solved the problems associated with poverty and underdevelopment, we are at the same time facing the challenges of rapid industrialization and modernization. In environmental health, biological contamination of food and water and indoor air pollution exist alongside the problems of chemical contamination and outdoor air pollution."

For years Mexico placed many of these problems on the backburner, but now the country is beginning to clean up its act. For example, last year the Mexican government banned DDT and chlordane, pesticides long outlawed in the U.S. And in 1997 leaded gasoline was eliminated. "There's been a dramatic shift in the attitude of those in power," says Leonora Rojas-Bracho, director of exposure and impact assessments in Mexico's Directorate of Environ-mental Health. "The previous and current regimes have made a real effort to address the effects of ambient pollutants and identify solutions. Now there's more money to conduct environmental measurements and policymaking research."

Since the early 1990s, Harvard School of Public Health scientists have taken a leading role in that research. Investigators in Environ-mental Health and other departments have worked to clearly identify levels and health effects of a wide range of pollutants and to devise strategies to reduce them. Along with counterparts in Mexico, they have initiated or contributed to several recent studies in three major areas: air quality management, industrial water pollution, and lead contamination.

BREATHING UNEASY
Clogged by traffic, teeming with industrial production, and surrounded by mountains, Mexico City collects more than its fair share of ozone and other ambient pollutants. But help may be on the way. A team of scientists in the School's Department of Environmental Health and Center for Risk Analysis are participating in a project aimed at devising a ten-year plan for improving the city's air quality. The project, part of a large program directed by MIT's Nobel Laureate Mario Molina, also involves researchers at MIT and several Mexican institutions. Jonathan Levy, S.D.'99, assistant professor of environmental health and risk assessment, has driven much of the School's effort.

Using data from Mexico City's air monitoring network and preliminary findings from the recent doctoral dissertation of Paulina Serrano, M.P.H.'95, the Harvard team determined that levels of exposure to particles, ozone, and other common air pollutants in Mexico City are high by U.S. standards, but roughly comparable to those in many megacities. Estimates of human health risks, computed using existing epidemiologic evidence from Mexico City and other large cities throughout the world, suggested that inhalable particles called PM10 and ozone are of greatest concern.

PM10 refers to particles with diameters of less than 10 microns, or ten millionths of a meter. "It's clear that these small particles are the worst because they penetrate further into the lung," says James K. Hammitt, associate professor of economics and decision sciences in the Department of Health Policy and Management. "When it comes to serious health effects, particles matter more than ozone." Produced by a variety of sources, including motor vehicles, power plants, and industrial emissions, PM10s can cause health problems ranging from respiratory infections to death. "Before this research and the recent World Bank study, the strong emphasis was on ozone," notes John Evans, co- director of the School's Program in Environmental Science and Risk Management. "It's been a real accomplishment to draw some attention to the role of fine particles."

The Harvard researchers estimated that a ten percent reduction in PM10 levels could decrease premature mortality in Mexico City by roughly 1000 deaths every year, or one percent of the overall baseline death rate. They also determined that Mexican citizens might be willing to spend as much as $3 billion annually to achieve that reduction. But this work is only a "back-of-the-envelope" risk assessment, cautions Hammitt. To obtain a more accurate evaluation of the economic merits of reducing air pollution levels, he and his colleagues plan to survey the general population, posing hypothetical questions about programs that would decrease pollution and increase taxes by varying amounts. At the same time, the researchers will explore the feasibility and likely benefits of new epidemiologic studies in Mexico that could improve the accuracy of future air pollution health risk assessments.

TROUBLED WATER
While ozone, particles, and other pollutants continue to dirty Mexico's air, industrial waste, agricultural runoff, and sewage contaminate its waterways. And lurking within this brew of water pollutants are a number of toxic metals such as lead and mercury. Found in lakes and reservoirs, these toxins threaten fisheries and pose health risks to human consumers of fish and drinking water. For more than ten years, Timothy E. Ford, associate professor of environmental microbiology, has examined the cycling of metal contaminants in Lake Chapala, the largest lake in Mexico. The lake is used for recreation and irrigation, as a fishery, and as the primary drinking water source for the city of Guadalajara, where about five million people reside. But it also serves as a receptacle for domestic and industrial wastes.

"We've been worried about pollution levels at Lake Chapala," says Rojas-Bracho. "Especially with all the industry in that area." Within Lake Chapala's watershed stand several battery and copper products manufacturers, paint and paint additive producers, petrochemical refineries, and iron and bronze foundries. Ford and colleagues from Baylor University recently found mercury concentrations in a minnow-like fish called chirostoma that far exceed U.S. and international health safety guidelines. They also discovered high levels of arsenic, cadmium, and lead in parts of the lake. Finally, they observed that clay particulate material in the lake absorbed toxic metals, retaining those metals in suspension with subsequent extraction to the municipal water supply. Based on the body of his research at Lake Chapala, Ford recommends that Mexico investigate mercury concentrations within other edible species in the lake and step up efforts to upgrade wastewater treatment and reduce industrial discharges in the watershed.

DREAD LEAD
Whether delivered by air, water, or food, lead contamination remains a serious environmental problem in Mexico, posing health risks that range from hypertension to birth defects. While leaded gasoline was only recently banned in the country, lead glazed ceramic food containers are still in common use, and control of other lead sources is not yet complete. Even if the country eliminated lead tomorrow, the problem of lead contamination would not evaporate. That's because 95 percent of lead gets stored in the bones of those exposed to it, and it can take decades before the lead clears out. This staying power can be a problem for pregnant and lactating women, whose bones release substantial amounts of calcium--and the lead stored within--into their bloodstream. The released lead may ultimately affect the health of a fetus or breastfeeding infant if lead concentrations in breast milk are sufficiently high. "There's been a lot of discussion in Mexico about how much lead transfers from mothers to babies through breast milk," says Rojas-Bracho. "Some mothers are concerned about whether or not to breastfeed."

In 1997, Environmental Health Professor Howard Hu, Dr. Mauricio Hernandez-Avila of the National Institute of Public Health in Mexico, and other Harvard and Mexico- based public health researchers demonstrated a clear linkage between high maternal bone lead levels and a lower birthweight in infants in Mexico City. "Babies born to women in the top 25 percent of bone lead levels were on average 156 grams lighter than those in the bottom quartile," notes Karen E. Peterson, associate professor of nutrition in the Departments of Maternal and Child Health and Nutrition and a co-investigator of the study. "That's a huge variation in birthweight."
This finding led to a subsequent study to determine if dietary calcium supplements can reduce the release of lead associated with bone turnover in lactating women. At a recent national meeting, Hu and his colleagues announced that a supplement of 1200 mg of calcium suppressed blood lead levels by 15 percent amongst women breastfeeding their infants. "These results suggest that calcium supplementation might afford a low-cost strategy for mitigating the effects on reproduction of lead stored in the bones of women," says Hu. If they can confirm their findings through additional studies, their work could induce clinics in Mexico to add calcium supplements during pregnancy and lactation.

Mark Dwortzan