When the temperature is above 100 degrees F, there’s no doubt an employer has to enact a “water, rest, shade” program for employees working outdoors. But what about somewhat lower, but still hot, temperatures? And how do humidity, direct sunlight and wind speed factor in?
An outdoor temperature of 80 degrees doesn’t have the same effect on workers in high and low humidity. Amount of exposure to direct sunlight also changes the risk.
As for guidance on these issues, it can be a bit sparse and varied, depending on the source.
Federal OSHA doesn’t have a heat stress standard (it does provide guidance). But four states do have regulations: California, Minnesota, Oregon and Washington.
Each of the four states uses different measurements of heat and other weather conditions to determine when to require employer action.
Let’s take a look at what the five agencies require or advise.
Heat index, web bulb temperature or something else?
OSHA references three measures of heat stress on its website:
- outdoor wet bulb globe temperature (WBGT)
- OSHA heat stress calculator, and
- National Weather Service heat index.
Of the three, the heat index is the best known because of its use by the NWS as well as other forecasting services and TV meteorologists. We’ll get back to the heat index later in this article.
The NWS defines WBGT as “a measure of the heat stress in direct sunlight, which takes into account: temperature, humidity, wind speed, sun angle and cloud cover (solar radiation).”
On its website, OSHA provides an outdoor WBGT calculator with software from UChicago.
To get the WBGT, you need:
- date, local time and time zone
- longitude and latitude, and
- current temperature, humidity, wind speed and barometric pressure.
OSHA’s heat stress calculator uses the WBGT, workload, acclimatization status, clothing and body weight to determine if a worker’s heat stress is above recommended limits.
What about states?
Here are the action levels for the four states with occupational heat stress regulations:
- For California’s Heat Illness Prevention Standard, a temperature of 80 degrees F triggers requirements
- Minnesota’s standard applies to indoor places of employment and uses a two-hour time-weighted average permissible heat exposure limit using WBGT
- Oregon adopted an emergency temporary standard through the end of September 2021 with two action levels: 80 and 90 degrees F, and
- Washington state’s regulation has action levels at 52, 77 and 89 degrees.
The good news about the state regulations is that they set simple, temperature action points.
Somewhat less good news is that the temperatures vary from state to state as do some details about the precautions businesses must take.
And federal OSHA’s guidelines, which apply to the other 46 states, territories and Washington, DC, suggests two methods of measuring employee heat exposure, one of which takes two calculations to arrive at a risk determination.
Heat Index
Confused? Don’t blame yourself. As important as heat stress is as a safety topic, the guidance on it is at times a little blurry.
There are easier ways to measure employee heat exposure. And some business owners knew about this decades ago. They were way ahead of their time.
True story. My mother worked as a secretary (administrative assistant in more modern terms) at a manufacturing plant in the 1940s.
The owner/manager would monitor temperature and relative humidity reports from the local airport on hot summer days.
He had his own formula for heat stress. When the sum of the temperature and relative humidity was more than 150, he closed the plant and sent the workers home.
(The heat index used today by the NWS wasn’t developed until 1979.)
The business owner’s formula is a bit imprecise — but better than nothing.
Good news for today’s managers and supervisors: A two-factor measurement is available, and finding it is no more difficult than opening an app on your phone.
The heat index, used by the NWS, combines temperature and relative humidity into a single value that indicates the apparent temperature – in other words, what it feels like.
OSHA and the National Institute for Occupational Safety and Health (NIOSH) have endorsed this measure of worker heat stress by creating a phone app that provides the heat index where you are automatically.
Search for “OSHA NIOSH Heat Safety Tool” in the Apple or Google app store.
When you first open the app, it will ask whether to allow access to your location.
If you allow access, the app will automatically determine the temperature and relative humidity for your location and calculate the heat index.
You can also put in another location (if you’re monitoring remotely, for example), or put in the temperature and relative humidity manually.
The app also provides the predicted heat index for the next several hours.
Besides giving the heat index, the app also shows the risk level: minimal, low, moderate, high or extreme.
Clicking on a button will provide more information about the risk level:
- water and shade
- emergency planning and response
- work and rest, and
- training.
Information on heat illness symptoms, first aid and frequently asked questions are also included on the app.
Heat Index 201
The app can give you most everything you need to know about occupational heat stress.
However, here is some additional important information (think of it as “Heat Index 201”):
- Most important: Heat index values are for shade and light winds. Exposure to full sunshine can increase the heat index by up to 15 degrees F.
- Know the difference between these NWS advisories: Heat Advisory (heat index expected to reach 100 to 109 depending on location); Excessive Heat Warning (heat index expected to reach or exceed 105 or 110 degrees depending on location)
- Some private weather services have their own proprietary heat measurements which, while useful, should not be considered the same as or mixed with the NWS heat index (example: AccuWeather’s Real Feel Temperature).
- What about indoors? OSHA (with the exception of Minnesota) has no indoor regulation, but it suggests indoor working environments be kept between 68 and 76 degrees F and 20% and 60% humidity.
