Beyond Temperature: How Humidity, Wind, and UV Affect Outdoor Workers
Air temperature isn't the only risk factor when it comes to heat stress. Humidity, windspeed, and UV levels all play a role.
2023 isn't over yet, but we've already experienced an unprecedented series of extreme climate events.
We had the hottest June on record, followed immediately by the hottest month ever recorded. For weeks, heatwaves and oppressive temperatures were the norm rather than the exception across multiple continents. During that time, the daily mean global surface temperature hit record-breaking levels. Meanwhile, North America had its worst wildfire season in recorded history.
(Learn more about Working Safely When the Air Quality Index Is High)
There's a lot more relevant data, but it all points in the same direction: it's hot, and getting hotter. Not only that, but it's reasonable to expect that these new records will be broken by even higher temperatures in the near future.
This is incredibly concerning. These temperatures are not simply uncomfortable; they're alarmingly close to being literally unbearable.
There are theoretical limits for how much heat exposure the human body can endure. At those levels, even workers who have been acclimatized to the heat will be at serious risk. When the temperature exceeds the human body's ability to cool itself, adverse health effects are inevitable and often severe. This is already a significant issue, especially when we consider the fact that heat injuries are believed to be underreported because they masquerade as other health problems, such as cardiac events. And it's only going to get worse as the heat intensifies.
OSHA responded to this ongoing issue with a new National Emphasis Program (NEP) focused on helping outdoor workers stay safe in hot weather, including definitions of the hazard, measurement criteria, and a framework for enforcement using specialized inspectors called Compliance Safety and Health Officers (CSHO). The program suggested methods like mandatory, scheduled, and frequent hydration breaks depending on the temperature hazard, monitoring for health effects, and moving tasks indoors where possible.
While that is commendable, we've also seen legislators rolling back legal protections at a time when they are most needed. In the midst of an unprecedented heatwave in Texas, legislation passed that, in effect, revoked the right of workers to take breaks at all. While the risks are evident, some lawmakers and business groups are still in denial about the severity of this hazard.
Until that changes, it is up to safety professionals and employers to go above and beyond the minimal requirements and take measures to protect their employees from heat hazards. And that begins by understand all the factors that can exacerbate heat stress and how to monitor them.
Windspeed, Humidity, and "Feels Like" Temperatures
When considering the risk to outdoor workers, air temperature is an important metric, but it's not the only one at play. If you have ever lived in a cold climate, for example, you'll know that the weather forecast will also factor in windspeed and humidity to generate a "feels like" temperature. This will often deviate significantly from the basic air temperature but more accurately reflect how hot it feels.
The same is broadly true of excessive heat. The impacts on the body will be different based on whether, for example, the air is completely stagnant or heavy winds are blowing. The body’s self-cooling mechanism (the evaporative cooling effect of sweat) is improved where there is greater air movement and impeded by high humidity and still air. Sweat will not evaporate as readily (or at all) in areas where the surrounding air is already saturated
Wet Bulb Temperature
One metric used to account for the effect of evaporation and humidity is called “Wet Bulb Temperature” (WBT). Using this method, we can roughly determined how much evaporation is possible. Water will evaporate faster into dry air, creating a larger cooling effect, whereas 100% relative humidity means no evaporation is possible at all, and the heat experience of the body (or “apparent temperature”) will be higher.
The method for measuring wet bulb temperature is simple in theory – a thermometer with a wet cloth cover is placed in the environment (or in the case of a sling psychrometer, swung around in the air). Where the air is saturated, the air temperature and reading will be equal; if evaporation has taken place, the WBT will be lower than the surrounding air.
One limitation of the sling psychrometer is that it simulates a very fast windspeed. We need to look at the actual windspeed to which workers will be exposed, since wind can replace the pocket of air around a worker with fresh air that has greater evaporative capacity. This makes windspeed a friend in the heat and a foe in the cold (where it contributes to the "windchill factor"). To accurately assess heat exposure, you have to account for this factor.
Wet Bulb Globe Temperature (WBGT) takes these additional factors into account using a thermometer or thermocouple enclosed in a black globe in order to calculate the mean radiant temperature. The formula for WBGT includes a weighting of the globe temperature, wet bulb temperature, dry bulb temperature, and the effects of visible or infrared radiation.
WBGT is the most comprehensive and involved method of determining the potential impact of the heat, but even this is oversimplified. It doesn’t account for the fact that clothing weight and cover is a variable, and full coverage is often required for workers. Being uncovered may help control body heat to some extent, but then there is the issue of the sunlight itself.
(Learn more in A Sweaty Situation: PPE, Hydration, and How to Manage Both)
Direct Sunlight and UV Exposure
Direct sunlight, of course, is an issue for more reasons than its effect on temperature. Sunlight is made up of UVA and UVB radiation, which contribute to the UV Index. That index is a measurement from ground level exposure to the different wavelengths of UV radiation, which have the capacity to cause adverse effects ranging from sunburn to cancer.
We have to consider those effects too when it comes to worker exposure. When spending excessive time under the sun, it is wise to cover up. Of course, then you have to consider how covering up cold contribute to body heat, which is a good encapsulation of how tough it can be to balance all these factors
There are complex methods of determining the potential for heat exposure, and limitations on what you can do to mitigate the problem. What is clear, however, is that this is an area that needs the focus of both innovators and lawmakers to protect those who work outdoors. Temperatures will keep turning up, but the problem is more complex than that. We need solutions that protect workers not just from simple air temperature, but from all the other factors that increase the risk of heat stress as well.