Hospitalizations for heart failure and acute myocardial infarction (AMI) spike in the winter—even in a city with a warm climate and modest temperature changes, new research suggests.

Previous studies—including one summarized by Cardiovascular Business in September—have found risk of cardiovascular disease increases in the winter in high-income countries with temperate climates. These countries experience vast temperature changes, which have been tied to increased risk, lead author Renato Kawisha Levin and colleagues wrote in PLOS One.

“Biological mechanisms linking low temperatures to higher cardiovascular risk include persistently higher sympathetic nervous system activation, uncontrolled hypertension, and an increased incidence of respiratory diseases,” they wrote. “Social and environmental mechanisms that hinge not only on low temperatures but on winter conditions more generally—such as shorter days, reduced physical activity, depression, and higher pollution levels—may also explain higher cardiovascular risk.”

But the researchers pointed out this seasonal finding hasn’t been fully researched in low- and middle-income countries with tropical and subtropical climates. To address this gap, they studied 76,474 hospitalizations for heart failure and 54,561 for AMI at public hospitals in Sao Paulo, Brazil, from January 2008 to April 2015.

Levin et al. found hospital admissions increased for both conditions in the winter, with increases of up to 30 percent for heart failure and 16 percent for AMI when compared to summer, which had the lowest figures for both diseases. Average winter temperatures were 17.5 degrees Celsius (63.5 degrees Fahrenheit) while average summer temperatures were 23.3 degrees Celsius (73.9 degrees Fahrenheit). The researchers noted monthly average temperatures were not significantly associated with hospitalizations, but other environmental factors may have played a role in the winter spike.

“Sao Paulo winters are characterized by low humidity, limited rains and a higher frequency of thermal inversions, which occur when cold air gets trapped near the surface and underneath a layer of warmer air,” they wrote. “Together, these conditions prevent the dispersion of pollutants such as carbon monoxide, nitrogen dioxide, sulfur dioxide and inhalable particulate matter that are associated with increased cardiovascular risk. Lower ambient temperatures, low humidity, and high pollution could also reinforce each other as they contribute to a higher incidence of respiratory diseases and influenza, with a consequent increase in cardiovascular risk.”

In addition, socioeconomic factors may have contributed to the increased risk of heart failure and AMI in the winter, Levin and coauthors said. Among Sao Paulo’s more than 11.2 million residents, 40 percent live in precarious housing conditions, the authors pointed out. This could leave them more susceptible to the worse pollution conditions of winter, although specific pollution data wasn’t available for the months of the study.

“Considering that mean temperatures were only moderately lower during the winter when compared to other seasons, our study raises the possibility that other factors related to disorderly urban occupation and urban poverty, such as a greater exposure to pollution and the lack of thermal protection in precarious housing, might also be associated with the seasonal increase in cardiovascular risk,” Levin et al. wrote. “This broader issue deserves further investigation.”