Iona Cheng, PhD, and her team will measure the effects of climate change on health

Anyone who’s lived in California during the past decade knows that heat waves and wildfire smoke have become both more frequent and more severe. Climate change has supercharged droughts, vast swaths of forest have died, and the resulting fires, coupled with extreme heat, have periodically turned the skies orange and driven residents inside. And although it’s fairly straightforward to estimate the effects of a given heat wave on human mortality rates, drawing conclusions about long-term trends is significantly more complicated.

Enter Iona Cheng, PhD, MPH, a professor of epidemiology and biostatistics at the UC San Francisco School of Medicine. Cheng and her team have received a $3.2 million grant from the National Institute on Aging to examine the long-term effects of climate change – specifically extreme heat and wildfire smoke – on adult life expectancy in a racially, ethnically and socioeconomically diverse population in southern California. She and her UCSF team will collaborate with principal investigators at other universities, including Anna Wu, PhD, of the University of Southern California and Jun Wu, PhD, of UC Irvine. Researchers from the University of Hawaii will also participate; all told, 17 scientists from the four universities will join the effort.

“We have three primary research aims,” Cheng said. “The first is to generate and characterize measures of extreme heat and exposure to wildfire smoke for the populations included in the study. The second is to look at the relationship between these measures and mortality, including deaths due to cardiovascular disease, respiratory disease and cancer, and then evaluate differences in associations by subgroup. The third aim is to learn more about the biological pathways related to DNA methylation and epigenetic age; that is, if exposure to these climate-change events is associated with biological aging, which could affect health outcomes.”

To study these complex interactions longitudinally, Cheng and her colleagues will use the epidemiological resources of the California Multiethnic Cohort Study (MEC), a large population-based study of about 112,000 men and women recruited largely from Los Angeles County, who were aged 45–75 at enrollment in 1993–1996. The cohort includes African Americans, Japanese Americans, Latino Americans and whites; it also records a wide range of socioeconomic status and related variables.

“One of the unique and rich resources the MEC provides is that from the time these people enrolled, we know where they’ve lived,” Cheng said. “That allows us to access secondary data sources about heat and wildfire smoke, and link those measures to the participants’ residential histories, which enables the study of long-term climate-change effects. In addition, this well-characterized cohort will allow us to look at whether the relationships of extreme heat, wildfire smoke and mortality differ between groups such as those of older and younger ages, those residing in coastal or inland areas, and those living in neighborhoods of low versus high socioeconomic status.”

The MEC data will allow Cheng and her team to correlate these residential histories with climate-change events occurring over about 25 years, in order to find patterns among, say, heat waves, cardiovascular mortality and neighborhood or socioeconomic characteristics. Since the MEC study began, 52,558 subjects – about 47 percent – have died. Of those, 19,868 died as a result of cardiovascular conditions, 3,604 of respiratory disease and 14,504 of cancer. Among other things, Cheng hopes to determine whether those ratios change with exposure to extreme heat or smoke, and to what extent other factors may play a role.

“The MEC provides rich information about lifestyle characteristics and comorbidities,” Cheng noted. “We can account for factors such as smoking, body mass index and physical activity, and we know that people with more comorbidities have a higher relative risk of death.” To help sort out such variables, Cheng’s multidisciplinary team includes scientists with expertise in demography, epidemiology, environmental health, climate science, aging, health disparities, molecular epidemiology and biostatistics.

“Jun Wu uses sophisticated models for estimating wildfire smoke, particulate matter, and various climate-change measures,” said Cheng, by way of example. “This gives us the novel methodology to estimate smoke exposure and measures of extreme heat, and it will also help factor in variables such as air pollution and how it may interact with climate-change measures. Are people at greater risk if extreme heat is combined with poor air quality, for example? We’d like to be able to answer some of those questions.”

Perhaps not surprisingly, Cheng and her colleagues expect to find that risk of death from cardiovascular disease, respiratory disease or cancer correlate with long-term or repeated exposure to extreme heat or wildfire smoke. If the data support the theory, there will likely be implications for environmental regulation, urban planning and similar fields.

“This research will inform policymakers, community members, city planners, physicians and others about risks and options going forward,” Cheng said. “We’d like to provide empirical evidence that helps reduce the negative health outcomes that may be associated with climate change.”