So far we have spent our time discussing all of the factors that affect our risk of heat related illness, from our age to the humidity. Now we finally get to talk about the things we have control over: the actions we take/or don’t take, the choices we make, and how we prepare. This will spill over into the next blog where we will closely look at a variety of cooling equipment and discuss their advantages and disadvantages.

Typically we use thirst and fatigue as key signals to let us know when we are overheating. Unfortunately, these are very poor signals because they are slow. By the time our body is aware of the issue it’s too late to immediately fix the problem, this is especially true for fatigue. Let's start with these two signals.

Thirst - Water Intake: 

Since we generally have a little bit more control when it comes to the signal of thirst, let’s talk about the recommended amounts of water we need to drink. When we sweat to expel heat we lose significant amounts of water and electrolytes that need to be replenished for our organs to do their jobs. 

The chart below shows the recommended amounts of water to drink based on the Wet Bulb Global Temperature and your activity level. WBGT is a very complicated temperature measurement which takes into account temperature, humidity, sun, and wind. For ease of use in everyday life, substitute the NWS Heat Index + Sun Exposure from the previous blog on environmental factors, for the WBGT in the chart below.



For example - A warm day in Rochester, NY on the NWS Heat Index comes out to 95F.  Imagine a gardener out in the sun tending their garden, they need to add 10F degrees. It’s already above the 90+F WBGT. Based on this chart they need to be consuming 1 qt of water/hour. We don’t know about you, but we don't measure every glass of water we drink but there are estimates we can use. Generally glasses you have around your house are pint glasses or slightly smaller. Since 2 pts =1qt, you need to drink approximately two glasses of water every hour to maintain a safe hydration level. 


 Drinking water or Gatorade alone will NOT prevent heat related illness.

There comes a time where you just need to STOP and rest.


You will notice that this chart stops at one qt/hr, as consuming too much water can leave your body without other vital nutrients. Drinking water or Gatorade alone will NOT prevent heat related illness. There comes a time where you just need to STOP and rest. This brings us to our next two factors.

Fatigue - Activity Level//Time Spent in the Heat:

Your activity level and time spent in the heat are very much intertwined with each other and so we should consider them together. It is important to remember that even at rest our bodies are producing enough heat to sustain our safe core body temperature of 98.6F. Anytime we are doing extra work our muscles increase the heat production substantially. That heat must be dissipated to keep you safe from heat stress and it gets much harder when the environment around you is hot. The chart below is a good guide for planning a rest schedule. Just be aware that it’s devised for workers who are acclimatized to their environment, under 40 years old, and hydrated.

The first question to always ask yourself when you see this chart is what exactly classifies as lightwork, moderate work, and heavy work. We all have different opinions on what that would be for each of us. Luckily scientists use our metabolic rate (calories you burn) while performing different activities to give us estimates for what sorts of activities fall into which categories. We have distilled their cumbersome charts into something more palatable:

  Workload/Activity   Level

  Activity Examples


  Sitting, thinking


  Sewing, writing, drawing, driving a car, a slow walk, standing.


  Continuous normal walking, picking fruits and veggies, hammering nails, cooking, raking, painting.


  Carrying loads, shoveling, landscaping, fast walking. (>4mph)

  Very Heavy

  Any activity done at/near maximum pace, climbing stairs


So, to help clarify, we will use Brad as an example. Brad (one of the co-founders of ThermApparel) is 33 years old and in average shape. He is adding a raised bed to his veggie garden. That would fall into the heavy activity category. The temperature is 86F and because he lives in Rochester, NY, he can assume the humidity will be 70%. This puts the heat index at 95F. To understand your heat index see last week's blog.

Since his veggies need sun to thrive he can assume that he will be in direct sunlight so that adds another 10F degrees. This puts the environment at 105F. Using the adjusted temperature in the graph above he should limit his activity to 25 minutes and then take a 35 min break. This will give his body the necessary time to cool down and recover before he starts again. This would also be a good time to drink that one qt of water to keep his hydration levels safe.

Your Clothing:

The next factor you have to consider is your clothing. We already do this somewhat by deciding to wear shorts or pants, long sleeve or short sleeve depending on the temperature. But for some jobs and activities, wearing more clothing is required. For example, Brad wears jeans when he is weed whacking to protect his legs. In the chart below we see how to increase your baseline external temperature if you are wearing different types of personal protective equipment (like Brad’s jeans).

When we don’t have this constraint we generally assume less clothing is better when it’s hot, as it increases airflow. The increased airflow over our bodies helps to get rid of excess heat. However, it is also important to consider the impact of direct sun on exposed skin. This can heat you up and can cause sunburn. A good example with how to deal with extreme heat and sun through clothing can be explored in the apparel of the Bedouin cultures. These nomadic people who traditionally inhabit the deserts regions of the Arabian Peninsula, North Africa, the Levant, and Mesopotamia have developed a clothing type uniquely suited for their hot, dry climate. They wear long flowing robes which protects their skin from harmful UV rays and allows for maximum air flow.  Even if it appears counterintuitive to a western audience. 

A Bedouin man wear a thoab, which is a long, dress-like cloth made of a mostly light fabric.


Cooling Gear:

Lastly there are many variations of cooling gear available, whether that is wicking clothing, cooling towels, cooling vests, fans, and umbrellas. Please note, cooling gear alone cannot completely compensate for all the risk factors we have discussed in these blogs. It is especially important to remember to stay hydrated and regardless of the cooling gear you pick, you will still sweat in the heat. The purpose of any cooling gear is to reduce the workload on your body's heart and other internal systems that get rid of excess body heat. This in turn will reduce your fatigue level during and after your activities. Also different cooling technologies work in different ways, you should pick cooling gear that is suited for you environment and to help offset your personal risk factors.

The purpose of any cooling gear is to reduce the workload on your body's heart and other internal systems that get rid of excess body heat.


 Different cooling technologies approach the problem from four main ways:

  1. Improving sweat evaporation - wicking clothing, cooling towels, some cooling vests

  2. Direct cooling - cooling vests, ice packs, cooling packs.

  3. Improving air flow - fans, clothing

  4. Reducing solar radiation - umbrellas, clothing

Each of these techniques have different values depending on where you live, your personal risk factors, and the activities you are doing. We will review the advantages and disadvantages of all of these in the next blog. Before we finish this series, we will provide you with a template for creating your own custom cooling plan.


   READ PART 6        

   Visit #HeatIQ Home      



**This web site is provided for educational and informational purposes only and does not constitute providing medical advice. The information provided should not be used for diagnosing or treating a health problem or disease, and those seeking personal medical advice should consult with a licensed physician. Always seek the advice of your doctor or other qualified health provider regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on ThermApparel’s website. If you think you may have a medical emergency, call 911 or go to the nearest emergency room immediately. Neither ThermApparel nor its employees, nor any contributor to this web site, makes any representations, express or implied, with respect to the information provided herein or to its use.

ThermApparel is a small business making big waves in the heat sensitivity world by designing the first lightweight, comfortable and invisible cooling vest, UnderCool. Check us out online, on our blog, or on FacebookTwitter, PinterestLinkedIn and Instagram.

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