By David Richards
Portable sensors and low-cost data offer a promising proxy method for calculating heat stress in direct sunlight, according to a recent study. The findings, published in May 2020 in GeoHealth, show data from local weather stations, NASA North American Land Data Assimilation System (NLDAS), and low-cost temperature and humidity sensors, can be used in combination to accurately predict Wet Bulb Globe Temperature (WBGT), a comprehensive measure of ambient temperature, humidity, wind, and sunlight used to estimate heat stress effects on the human body.
“We have known for a while now that WBGT is an accurate estimate of heat stress risk, especially in occupational settings,” said study author Julia Gohlke, Ph.D., associate professor of environmental health at Virginia Polytechnic Institute and State University and former NIEHS post-doctoral fellow. “This study estimates WBGT from different types of easily accessible data sources. Those can be satellite derived, deployed low-cost sensors for neighborhood-level temperature and humidity measurements, and data generated at weather stations.”
As global temperatures rise and incidences of extreme heat and heat waves increase, exposure to these events can result in occupational illness and injury. WBGT, which takes into account direct sunlight as opposed to the heat index, is widely used across the U.S. government within military, labor, and public health agencies to determine appropriate exposure levels and work-rest guidelines for outdoor workers and is similarly used around the world.
An Estimation of WBGT
The research team conducted the study in Alabama, as the southeastern region of the U.S. has experienced an increase in heat waves since the 1980s. The researchers picked three sites in an urban setting in downtown Birmingham, suburban Birmingham, and rural Wilcox County, to account for microclimate impacts.
“WBGT is a more sophisticated way of understanding how heat is affecting physiology,” said study co-lead author Benjamin Zaitchik, Ph.D., associate professor in the Department of Earth and Planetary Sciences at Johns Hopkins University. “However, it is not conventionally measured at weather stations and requires variables that are not always available to local unmonitored environments.”
The researchers collected field data from portable iButton hygrometers, a low-cost and widely available tool which measures temperature and humidity, and assessed data from local weather stations and NLDAS. The hygrometers and weather stations provided on-the-ground meteorological measurements, while NLDAS provided additional estimates of solar radiation and wind. Together, these data sources were compared to Kestrel Heat Stress Trackers, a more costly tool designed to measure WBGT, which were installed at select locations.
The researchers found the WBGT estimates from the Kestrel monitors and the combined data sources statistically indistinguishable at two of the three sites. “We were able to measure WBGT quite well,” said Gohlke. “We found the measurements from the iButton hygrometers improved the estimates, suggesting that microclimates within neighborhoods do have an appreciable effect on the estimates of WBGT.”
In addition, the research team examined occupational heat stress thresholds for a hypothetical work-rest schedule using the U.S. Centers for Disease Control and Prevention National Institute for Occupational Safety and Health-recommended heat stress exposure limits and appropriate work‐rest periods. The study found the WBGT estimation with the iButton hygrometer data suggested a greater number of hours requiring a higher rest to work ratio compared to using local weather station data alone.
“Physiologically, your body wants to maintain a core body temperature to keep everything functioning correctly,” said Gohlke. “When the ambient WBGT of your environment is high, your body cannot use sweating as effectively to keep that core temperature. A robust WBGT estimation method for local environments can inform interventions related to heat stress and its health impacts.”
Implications for Global Environmental Health
The study is part of a larger project led by Johns Hopkins University and Virginia Tech, including Gohlke and Zaitchik, to collect microclimate data in determining heat stress and exposures for different local environments. Their subsequent studies in Alabama work with urban and rural communities to dive further into neighborhood level burden of heat stress, including one study that examines an individual’s outdoor physical activity relative to their experienced heat index.
In addition, Gohlke and Zaitchik contributed to a study on temperature and heat stress in informal settlements in different parts of Nairobi, Kenya. The study used similar iButton sensors to find variations in temperature when comparing measurements from the neighborhood sensors with measurements from the Kenya Meteorological Department at the Dagoretti weather station, only kilometers away from each other. The study found the heat index in the largest informal settlement in Nairobi regularly exceeded, by several degrees, temperature measurements taken at the formal observation station nearby, reaching levels associated with increased mortality in warmer seasons. This observation adds to the existing literature on heat and urbanization, while emphasizing the importance of research on microclimates in large urban areas and the associated health risks.
As global temperatures continue to rise, these studies are providing a better understanding of how prolific the heat is and its associated health outcomes. “Heat is the deadliest climate hazard in the U.S. and many places around the world,” said Zaitchik. “As we are seeing under climate change, that is only increasing. We are dealing with a situation where countries are experiencing temperatures that make physical activity difficult, especially in vulnerable communities.”
Julia Gohlke, Ph.D., participated in this year’s NIEHS Global Environmental Health Day to present her findings in central Alabama. Read the GEH Day feature article to learn more.
Carter, A. W., Zaitchik, B. F., Gohlke, J. M., Wang, S., & Richardson, M. B. (2020). Methods for estimating wet bulb globe temperature from remote and low‐cost data: A comparative study in Central Alabama. GeoHealth. 2020 May; 4(5): e2019GH000231. 10.1029/2019GH000231