How a Sunny Beautiful 86°F can be Deadly Hot!

Understanding environmental risk factors is crucial for preventing heat stress, whether working outdoors, exercising, or simply spending time in hot environments. Below, we summarize the five main environmental factors contributing to heat stress and offer actionable tips for managing your risk.

Key Environmental Risk Factors for Heat Stress

• Temperature

• Humidity

• Direct Sunlight

• Other Nearby Heat Sources

• Air Movement

1. Temperature

Ambient temperature is the most obvious factor influencing heat stress. When temperatures rise above 80°F (27°C), the body must work harder to expel excess heat, and productivity can decline. This threshold may be even lower for individuals with higher personal risk—such as older adults, those with certain health conditions, or those taking specific medications. It’s important to note that official temperature readings are typically taken in the shade with dry-bulb thermometers, so actual “feels like” temperatures in direct sun or near heat sources may be significantly higher.

2. Humidity

Sweat does not cool the body; evaporation of sweat cools you. No evaporation means no cooling for your body. High humidity impairs the body’s ability to cool itself through sweating. When the air is saturated with moisture, sweat evaporates less efficiently, trapping more heat in the body. High humidity can dramatically increase the risk of heat-related illness even at moderate temperatures. For example, at 70°F with 96% humidity, the risk of heat illness is much higher than the temperature alone would suggest. The heat index, which combines temperature and humidity, accurately measures perceived heat stress. People die from overheating on a 70° days because the humidity is 98%.

For example: In my climate zone for humidity is 70% and the ambient temperature is 86°F, that means to your body it is now 95°. Now a worm day is a potently dangerous hot day.

Cross-reference with the relative humidity row you found earlier. For our example, we will use 70% as it is the middle humidity based on the location we are in. 86°F with 70% humidity gives us a heat index of 95°F or +9°F over the stated temperature.  

3. Direct Sunlight

Exposure to direct sunlight can significantly increase heat stress. Compared to shaded areas, sunlight raises the heat index by as much as 13.5°F. If you plan to be in open, unshaded spaces—such as at a sports field or on a boat—add about 10°F to the reported temperature to estimate your actual risk. Picture this: You're on a soccer field under direct sunlight. The temperature on your phone says it's a nice day, sunny 85°F, but suddenly, it feels like or is 98°F.  What was once a pleasant day instantly turns dangerously hot by stepping into the sunlight.

4. Other Nearby Heat Sources

Have you ever felt like the amusement park is the hottest place ever? Here is why. Heat sources like machinery, engines, asphalt, and even urban infrastructure (buildings and pavement) can raise local temperatures by several degrees. Metropolitan areas, in particular, can be 2–5°F hotter on average and up to 12°F hotter at night due to the “urban heat island” effect. Always consider these additional sources when planning your cooling strategies. 

5. Air Movement

Air movement is a critical factor in reducing heat stress. Breezes or fans help evaporate sweat and cool the body.  However, heavy clothing or personal protective equipment (PPE) can block airflow, negating this benefit. Wear lightweight, breathable clothing whenever possible and ensure good ventilation in hot environments.

How to Manage Environmental Heat Risk

• Monitor all five environmental factors—not just temperature—when assessing your risk for heat stress.

• Use tools like the heat index or, for workplaces, a Wet Bulb Globe Temperature (WBGT) monitor to get a comprehensive view of heat risk.

• Plan for extra precautions (e.g., hydration, rest breaks, and cooling gear) when multiple risk factors are present.

• Adjust your activities based on local conditions and your risk profile.

By understanding and monitoring these environmental risk factors, you can take proactive steps to prevent heat stress and stay safe in hot environments. For more tips and resources on heat safety, check out the next part of our series, where we’ll cover controllable factors like activity level, hydration, and clothing.

Let’s combine it with what we like to call Heat + Math.

We have an exciting day. Today we are going to an amusement park. Yeah  

• Ambient air temperature 86°F

• Humditiy for Florida 80%

• In direct sunlight, yes, plus 13°

• Standing on asphalt plus 5°

• Wind 5mph

Heat Math 

86°F+80%humidity = heat index 100°F + direct light 13°+ asphalt 5°- 5mph wind 3° = 115°F 

It’s not the rides or the corndogs that make you feel exhausted the next day; it's the heat.

Stay informed. Stay cool. 

Increase your #HeatIQ.

 

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