Now that we have a better understanding of our personal risk factors, let's talk about the factors being imposed on us by our surroundings.

Now that we have a better understanding of our personal risk factors, let's talk about the factors being imposed on us by our surroundings.

There are 5 main environmental factors: temperature, humidity, direct sun, other nearby heat sources, and air movement. We will discuss in detail each of these factors and how they should be considered in relation to each other and a comprehensive cooling plan.

So let's start at the top. Temperature is the big one, the one we all think about as part of our daily routine. The most important number to remember for the average middle-aged person is 80°F. Over 80°F we begin to become less productive as our body has to work harder to expel excess heat. However, depending on your personal risk factors, you may want to lower this threshold temperature by 5-6 degrees. As we mentioned in earlier posts, your age, fitness, and medications can already reduce your body's ability to expel heat. It's important to note also how temperature is recorded by your friendly local weather person. Temperature is taken with a dry-bulb thermometer, meaning it is exposed only to the air (no moisture) and in the shade (no sun). This means that in many places what it “feels like” will be much hotter than the temperature reading. 

This gets us to factor #2 Humidity. Humidity causes a substantial increase in the risk of heat-related illness due to its effect on the body's most potent cooling process, sweat. Humidity is a measure of water vapor present in the air around you. The more water already present in the air makes it harder for sweat to evaporate off your skin, thus, making it harder for the heat to escape to your environment.

To highlight how significant a risk humidity can be, we want to bring up an unfortunate event in Brooklyn, NY. During the 2021 Brooklyn Half Marathon, thirty-two-year-old David Reichman of Brooklyn collapsed after crossing the finish line. Although Reichman received medical attention, he later died in the hospital. Fifteen other runners were hospitalized with injuries linked to the high temperatures and humidity during the race. At the time of the injuries, the temperature was around 70°F but with 96% humidity, the risk of heat-related illness increased dramatically. Something it seems many people were not prepared for. 

During the summer months, especially if there is an upcoming heat wave, you may hear the weatherperson bring up the heat index. The heat index incorporates humidity with the temperature to give you a better idea of what it  “feels like.” For a significant portion of the US, humidity can raise “feels like” temperatures 7-16 degrees on a normal day.

Rather than trying to find the heat index every day,  Estimate the heat index considering where you live. For example in July in Rochester, NY we usually add 10 degrees to the temperature on a warm day to account for the humidity. This is relatively easy to do and we will show you how to create your own rule of thumb for your location: 

  1. Find where you live on the map below. This is a chart of the average humidity for the continental US in July. 

  1. Match the color of your location to the relative humidity using the key on the right of the map.  For instance, ThermApparel is in Rochester, NY we are in the Orange zone so 66-75% relative humidity.

  2. Determine an average temperature for your location.  The average high temperature for Rochester, NY in July is 82°F.  This means our bodies should be relatively acclimatized to 82°F.  We will use 86°F as my temperature for this step as this is when we start paying attention to being careful in the heat.

  3. Find that temperature on the top row heading of the NWS Heat Index chart below.

What is the heat index?

  1. 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.  

Looking at the heat index chart you may notice that along the relative humidity rows the heat index increases dramatically as temperatures increase. From our example, on an 80°F day that only adds 3F. Not bad.  But on a 90°F day, it adds 15°F and puts us into the danger zone. Make sure you are aware of this for your location and are prepared. Ask yourself at what dry bulb temperature am I in the extreme caution zone and what is my plan for this? At what temperature is my location in the danger zone and how do I augment my Cooling Plan for this situation? 

Another major factor not considered in our weatherperson's forecast is direct sunlight exposure. While we have all experienced this before and know the benefits of shade. It is surprising how large the temperature difference is in the sun vs the shade. Direct sunlight can increase the heat index up to 13.5 degrees. So, if you are planning on being out somewhere that has ample shade, a nice wooded park, or a large stadium with covered seating you can probably use the heat index temperature above. But, if you are planning on being somewhere very open like your kid’s soccer game, or out on a boat you need to make sure you add about 10 degrees to the temperature.

To go back to our example. We want to go out on Rochester’s great lake, Lake Erie. It's 86°F out +10 degrees for my rule of thumb based on our location's relative humidity +10 degrees for no shade. That puts the actual temperature at 106F!  86°F (temperature) + 10°F (humidity rule of thumb) + 10°F (no shade) = 106°F

We are now to the last of the “add-on” factors, ONHS (Other Nearby Heat Sources). This is especially important for workers (think a steel mill with all that molten steel, or a food truck with the grills going and generators running). The temperature degree differences here are wide-ranging so we won’t list them all out but you should consider them in your plan. 

Another source of heat many of us don’t consider, but it affects us daily, is the urban microenvironment (buildings, asphalt, concrete). It’s well documented that cities are hotter than their surroundings due to the thermal mass of infrastructure (+2-5°F degrees average, up to +12°F degrees hotter in the evenings). You may have felt the effects of this when standing in a hot parking lot. 

There can be times when these external sources can provide cooling as well. This is common around large bodies of water like oceans where a current of cooler water makes the local environment cooler than it otherwise would be. Or at higher elevations in mountains, where lower air pressure keeps things cooler.

The last environmental factor is a good one, it reduces our heat stress. And that is air movement. Ever been to the beach? High temperature, check. High humidity, check. High direct sunlight, check. And yet many times it feels pretty comfortable, doesn't it? That is due to air movement. It’s usually pretty breezy at the beach and this air flow over our skin helps our body expel excess heat without more sweating. One thing to also be aware of concerning air movement is your clothing choices. For example, regardless of the environmental air movement, if you are wearing heavy clothes, PPE, or anything that prevents the flow of air directly over the skin, this effect will be lost.

That's all for today. Make sure you take a good look at the NWS Heat Index chart and know your dry bulb danger zone temperature. This will be very important as we build out your Cooling Pan and increase your #heatIQ in future installments. We can't wait to have you back next week when we discuss the factors that you have control over, like activity level, hydration, and clothing.


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