“When is it too hot to work or practice?” sounds like a simple question. But the answer is rarely as simple as one number on a thermometer.
Heat stress risk depends on more than temperature. Humidity, sun exposure, wind, physical exertion, clothing or equipment, acclimation, and time all affect how much heat the body has to manage. A practical prevention program accounts for all these factors while giving leaders structured guidance on when to modify, delay, or stop activity.
In practice, those decisions look different depending on the setting and the people responsible for making the call. For an operations or safety manager overseeing a worksite, that may mean rotating crews, adding recovery breaks, moving strenuous tasks to a cooler part of the day, or suspending certain activities. For a school coach or athletic director, it may mean shortening practice, reducing conditioning intensity, adjusting equipment, delaying a game, or moving activity indoors.
But the goal is always the same: to make heat-related decisions consistently and appropriately for the conditions people are actually facing.
In this guide, we’ll:
Anyone who has spent time outside on a hot day knows that a dry 90 degrees feels very different than a humid 90 degrees. Air temperature matters, but it does not fully explain how heat affects the body during work, practice, or competition.
A sunny, humid afternoon with limited wind can create a very different level of heat stress than the same air temperature in shade with a breeze. A light activity may be manageable under conditions that would be unsafe for heavy labor, intense conditioning, or practice in heavy pads. A worker who has spent two weeks getting acclimated to hot conditions may not face the same risk as someone returning from time away. An athlete in early preseason may not respond the same way as one who has gradually built up exposure.
That is why “too hot” should be treated as a risk question, not just a temperature question.
Heat stress risk rises when the body has a harder time shedding heat than it does producing or absorbing it. That can happen because of the surrounding environment, the intensity of the activity, the clothing or equipment being worn, the length of exposure, or how prepared a person is for hot conditions.
Those influences fall into three broad categories: the surrounding environment, the activity and exposure involved, and the readiness of the person doing the work or practice.
Start with the environment. Air temperature matters, but it is only one part of the heat stress picture. Humidity can make sweating less effective. Direct sun adds radiant heat. Low wind limits air movement across the skin. Together, those conditions can make the same air temperature feel and function very differently from one setting to another.
Heat index goes a step further by accounting for the combined effect of heat and humidity. But it still does not capture enough of the conditions that affect heat stress risk during physical activity. That is why the military, workplaces, and school athletic programs have increasingly turned to Wet Bulb Globe Temperature, or WBGT, as a more complete metric for guiding activity decisions.
WBGT combines temperature, humidity, radiant heat from sunlight, and wind into a single measurement. That makes it especially useful when people are not just exposed to hot weather, but also generating body heat through strenuous work, conditioning, practice, or competition. AEM’s WBGT explainer covers that metric in greater detail, including how it is calculated and how it differs from heat index.
Environmental conditions are only part of the picture. There’s also the intensity of activity, the kinds of equipment and gear that are being used, and the duration of exposure.
Acclimation can also change a person’s heat risk. A worker who is new to the job, returning after time away, or ramping up during the first hot stretch of the season may face higher risk than someone who has gradually adapted. The same principle applies to athletes early in preseason, especially when practice intensity and equipment use increase before the body has adjusted.
Other individual factors can further affect vulnerability, including hydration, fatigue, physical fitness, health conditions, medications, previous heat illness, and age. Those factors do not replace environmental monitoring, but they may affect when and how leaders respond.
For prevention planning, the point is to understand how all these factors combine to create risk. A humid afternoon, a turf field in direct sun, heavy pads, repeated conditioning drills, and athletes early in preseason create a different risk profile than light activity in shade with a breeze. A construction crew doing high-exertion work in PPE on a paved site faces a different risk profile than workers doing lower-intensity tasks in a shaded area.
| Heat Risk Type | Specific Risk Factors | Why They Matter |
| Environment | Air temperature, humidity, sun exposure, wind, radiant heat, surface conditions | These conditions affect how much heat people absorb and how effectively the body can cool itself |
| Activity & Exposure | Work or practice intensity, duration, uniforms, helmets, pads, PPE, other gear | Physical exertion increases internal heat load, while equipment and longer exposure can make cooling harder |
| Individual Readiness | Acclimation, hydration, fatigue, physical fitness, health conditions, medications, previous heat illness, age | People do not respond to heat the same way, so the same conditions can create different levels of risk |
A heat stress prevention program is only as useful as the information guiding it.
Regional forecasts and distant weather observations can be helpful for planning, but they may not reflect the conditions people are actually experiencing. A nearby airport observation may not capture conditions on a turf field, a construction site surrounded by concrete, a paved loading area, or an industrial facility with indoor heat sources.
That is why local monitoring matters. A properly sited weather station can help organizations assess heat conditions where work, practice, or competition is actually happening. With the right sensors and software, weather stations can support real-time WBGT calculations. That reduces the need for repeated handheld readings throughout the day. Handheld meters often need time to equilibrate after being moved into position, which can make repeated readings harder to manage as conditions change.
Once a prevention program identifies the factors that affect heat stress risk, the next step is deciding how work or practice should change. Although specific actions need to fit their setting, the same broad prevention logic applies across worksites and athletic programs.
As thresholds for action are approached, leaders may need to watch conditions more closely. That might mean checking updated WBGT readings, watching for changes in sun or wind, or reassessing conditions before the next work period, drill, practice, or event.
But the larger purpose of monitoring is to know when the plan needs to change. In practice, those changes usually fall into three broad response types.
At an initial action threshold, work or practice may continue with added protections. For worksites, that may mean more frequent recovery breaks, shaded or cooled rest areas, supervisor check-ins, buddy systems, hydration reminders, or modified work/rest cycles. For athletic programs, it may mean longer rest periods between drills, more frequent hydration breaks, reduced conditioning volume, or closer attention to athletes who are new, returning, or not yet acclimated.
At higher thresholds, leaders may need to change the activity itself. On a worksite, that could mean rotating crews more frequently, reducing the intensity or duration of strenuous tasks, moving work into shade when possible, or adjusting PPE-related work where safety requirements allow. In athletics, it could mean reducing practice intensity, shortening practice duration, changing the order of drills, limiting conditioning, increasing recovery time, or changing how equipment is phased in.
When added protections and activity modifications are not enough, the appropriate decision may be to delay, reschedule, suspend, or stop the activity. A high-exertion task may need to move to a cooler part of the day. A practice may need to be rescheduled for early morning or evening. A game, tournament, or outdoor event may need to be delayed, suspended, or canceled.
| Response Type | What Changes? | Examples |
| Add Protections | Air temperature, humidity, sun exposure, wind, radiant heat, surface conditions | More recovery breaks, shade or cooling, hydration reminders, buddy systems, closer check-ins |
| Modify Work or Practice | Work or practice intensity, duration, uniforms, helmets, pads, PPE, other gear | Reduced intensity, shorter duration, fewer conditioning reps, crew rotation, shaded task location, equipment adjustments |
| Delay, Reschedule, Suspend, or Stop | Acclimation, hydration, fatigue, physical fitness, health conditions, medications, previous heat illness, age | Move work to cooler hours, reschedule practice, delay games, suspend or cancel events |
The key is to define these options before hot conditions arrive. A supervisor or coach should not have to invent the response in the middle of a hot afternoon. Organizations should predefine who can make the decision, what conditions trigger escalation, and how changes will be communicated. That makes leaders less likely to delay action because the decision feels disruptive or uncertain.
Automated alerts can support that process by turning monitoring into timely prompts for action. Rather than relying on a coach, supervisor, or safety manager to remember when to check conditions, a system can automatically notify the right people when heat conditions make it necessary to adjust activities.
Heat stress prevention is not only a matter of discretion. In many settings, regulations, policies, and governing-body guidance are raising expectations for how organizations manage heat risk.
NATA: National Athletic Trainers’ Association (NATA) guidance recommends preseason heat-acclimatization policies and hot-weather event guidelines based on activity type and WBGT, with gradual heat acclimatization over 7 to 14 days.
The details vary by jurisdiction, governing body, and organization. But the takeaway is clear: expectations for managing heat stress risk are rising. Leaders need to understand the rules, policies, and guidance that apply to their setting — and make sure their prevention programs are ready to support timely, consistent decisions when heat conditions change.
After leaders account for the factors behind heat stress risk, the next question is whether their prevention program can guide decisions when conditions change. A readiness review should ask practical questions like these:
There is no single temperature that answers the question, “When is it too hot to work or practice?”
The safer answer is more complex. Supervisors, coaches, and safety managers need to account for how heat conditions, activity demands, equipment, exposure time, and individual readiness all affect the people doing the work or practice. Leaders need all those factors to safely determine whether current conditions call for normal activity, added protections, modified work or practice, delay, suspension, or cancellation.
That complexity is exactly why organizations need a plan before conditions become dangerous.
When leaders have local environmental information, predefined action guidance, clear authority, and reliable communication, they are in a much stronger position to address the real issue behind “when is it too hot?” They can make consistent, defensible decisions that hold up under scrutiny.
If your organization needs a clearer way to monitor and act on heat conditions before they become dangerous, AEM can help.