Can high heat impair the immune system?

Inflammation is a normal part of the body’s defence against injury or infection, however, an inflammatory response that is longstanding — lasting weeks to months — or that occurs in healthy tissues is damaging and plays a key role in the build-up of plaque in the arteries. This may lead to atherosclerosis. Heat waves are known to promote inflammation, however, more study is needed.

“Most research only considers temperature as the exposure of interest, which may not be adequate to capture a person’s response to heat. In our study, we used alternative measurements of heat in relation to multiple markers of inflammation and immune response in the body to investigate the short-term effects of heat exposure and produce a more complete picture of its health impact.” said lead study author Daniel W. Riggs, Ph.D., an assistant professor of medicine at the University of Louisville in Kentucky.

In the study, Participants visited various sites during the summer months for a blood test, and were analysed for multiple markers of immune system function. The researchers then examined associations between the markers of immune system function and heat levels, including temperature, net effective temperature (which factors in relative humidity, air temperature and windspeed) and the Universal Thermal Climate Index (UTCI) on that day, which is a thermo-physiological model that factors in temperature, humidity, wind speed and ultraviolet radiation levels, and used to evaluate participant’s physical comfort.

The analysis found:

1.For every 5-degree increase in UTCI, there was an increase in the levels of key markers of inflammation: monocytes (4.2%), eosinophils (9.5%), natural killer T-cells (9.9%) and tumor necrosis factor-alpha (7.0%) in the blood. These immune molecules indicate activation of the body’s innate immune system, to protect against pathogens and injury.

2.A decrease in B-cells (-6.8%), indicating the body’s adaptive immune system that remembers specific viruses and germs and creates antibodies to fight them, was lowered.

3.A lesser impact on the immune system was found when heat was measured by average 24-hour temperature or by net effective temperature, which incorporates humidity and wind but not sunshine.

“Our study participants only had minor exposure to high temperatures on the day of their blood test, however, even minor exposure may contribute to changes in immune markers. With rising global temperatures, the association between heat exposure and a temporarily weakened response from the immune system is a concern because temperature and humidity are known to be important environmental drivers of infectious, airborne disease transmission. Thus, during the hottest days of summer people may be at higher risk of heat exposure, they may also be more vulnerable to disease or inflammation.” said Riggs.

Reference: Heat exposure may increase inflammation and impair the immune system; American Heart Association Epidemiology and Prevention