Most occupational safety and health professionals are familiar with the National Weather Service’s (NWS) heat index chart. It’s a well-known graphic that is often referenced when discussing heat-related illnesses at work.

In a ruling on July 15, 2020, an administrative law judge of the independent Occupational Safety and Health Review Commission (OSHRC) held that “OSHA failed to show that a document the agency used to prosecute employers in heat stress cases—the NWS heat index chart—has a scientific basis.”

The ruling is expected to reverberate widely, as OSHA and its lawyers have many times used the chart as evidence that employers violated the General Duty Clause of the OSH Act, according to a blog post from law firm Ogletree Deakins, part of the USPS legal team.

The Postal Service Cases

The ruling was issued in relation to five heat stress citations OSHA issued to the U.S. Postal Service (USPS) in 2016 and 2017 for alleged general duty violations in San Antonia and Houston, TX; Des Moines, IA; Charleston, WV; and Benton, AR. OSHA introduced the NWS heat index chart into the trial records and pointed to it as evidence.

In building a defense in the case, the legal team specifically looked at “whether the chart’s assertion that a heat index value in the orange zone or above represented a ‘danger’ was supported by scientific evidence.”

As part of this research, the team reviewed the history of what are considered the two layers of information that make up the NWS index. The first layer—the number grid—is based on research by R.G. Steadman, a professor at Colorado State University, which was published in the 1979 article, “The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science." Steadman's goal was to identify “a single measure of the combined effects of high temperature and humidity. . . . just as a combination of wind and low temperature is referred to as ‘windchill’ . . . the ‘apparent’ temperature . . . is the temperature which a given combination of dry-bulb temperature and vapor pressure ‘feels like’ to the typical human.” He made no mention that a particular heat index value would cause a certain physiological effect or pose a certain risk.

However, the chart’s so-called second layer adds — via color-coding and a legend —  information on physiological effects, such as “Caution” or “Extreme Caution,” and risks, such as “Danger” or “Extreme Danger.” Information on the source of this color coding and legend is scarce, but it likely is based on information from a 1981 article by Robert Quayle and Fred Doehring in Weatherwise, a popular magazine on weather and climatology. That article contains a small chart with a basic heat index, but this chart is not attributed to any source.

“USPS argued that the second layer of the NWS’s heat index chart lacked a scientific basis and should be disregarded,” the blog post states. “USPS also pointed out that the co-authors of the [1981] article [were] a meteorologist and a climatologist, [neither] known to have any qualifications in human physiology.”

The July 2020 Decision

Administrative Law Judge Sharon Calhoun agreed with USPS that no evidence was presented to establish the scientific basis for the risk categories depicted on the second layer of the NWS heat index chart. She found that OSHA had failed to provide any "supporting data . . . for why the levels of risk [indicated by the chart’s color coding and legend] are attributed to their respective temperatures,” and subsequently vacated all five citations.

As of this writing, the agency has not asked the full commission to review the judge’s decisions. “Unless review is sought and granted, and the judge’s decisions are adopted as the commission’s own, they will not be binding on any other judge or the commission," the post explains. "If OSHA does seek review and it is granted, the judge’s findings about the chart are highly unlikely to be challenged, let alone overturned.”

Given Calhoun's standing as a widely respected judge highly experienced in occupational safety and health law, the post concludes that her decisions "likely spell the end of OSHA’s ability to rely on the NWS heat index chart.”