Methods to calculate the heat index as an exposure metric in environmental health research

Abstract

Background: Environmental health research employs a variety of metrics to measure heat exposure, both to directly study the health effects of outdoor temperature and to control for temperature in studies of other environmental exposures, including air pollution. To measure heat exposure, environmental health studies often use heat index, which incorporates both air temperature and moisture. However, the method of calculating heat index varies across environmental studies, which could mean that studies using different algorithms to calculate heat index may not be comparable.

Objective and methods: We investigated 21 separate heat index algorithms found in the literature to determine a) whether different algorithms generate heat index values that are consistent with the theoretical concepts of apparent temperature and b) whether different algorithms generate similar heat index values.

Results: Although environmental studies differ in how they calculate heat index values, most studies' heat index algorithms generate values consistent with apparent temperature. Additionally, most different algorithms generate closely correlated heat index values. However, a few algorithms are potentially problematic, especially in certain weather conditions (e.g., very low relative humidity, cold weather). To aid environmental health researchers, we have created open-source software in R to calculate the heat index using the U.S. National Weather Service's algorithm.

Conclusion: We identified 21 separate heat index algorithms used in environmental research. Our analysis demonstrated that methods to calculate heat index are inconsistent across studies. Careful choice of a heat index algorithm can help ensure reproducible and consistent environmental health research.

Figure 1

Distributions of daily temperature and relative humidity in U.S. state capitals in 2011 (A) and data from Steadman’s original apparent temperature table (B) (Steadman 1979a), which has been reformatted to correspond with the weather distribution graph and gives apparent temperature values in degrees Celsius. For the distribution graph (A), darker areas indicate more days with the given weather, and white indicates no days with those weather conditions in the U.S. state capitals in 2011. Weather conditions covered by Steadman’s table for air temperature and relative humidity are indicated by the dotted line. Data from Steadman (1979a), ^©American Meteorological Society, are used with permission.

Figure 2

Distributions of daily temperature and dew point temperatures in U.S. state capitals in 2011 (A) and data from Steadman’s original apparent temperature table (B) (Steadman 1979a), which has been reformatted to correspond with the weather distribution graph and gives apparent temperature values in degrees Celsius. For the distribution graph (A), darker areas indicate more days with the given weather, and white indicates no days with those weather conditions in the U.S. state capitals in 2011. Weather conditions covered by Steadman’s table for air temperature and dew point temperature are indicated by the dotted line. Data from Steadman (1979a), ^©American Meteorological Society, are used with permission.

Figure 3

Algorithm used by the NWS online heat index (HI) calculator (NWS 2011) to determine heat index based on air temperature in degrees Fahrenheit (T) and relative humidity in percent (H).

Figure 4

Daily differences between heat index and air temperature for each day in 2011 for five U.S. cities. Color shows heat index minus temperature for that day in 2011 in the specified city. Lighter colors indicate that heat index and air temperature were very similar. Darker red (blue) indicate heat index was higher (lower) than air temperature. The figure shows the difference in temperatures, not absolute temperatures.