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 Part 6: Creating a Cooling Toolbox 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 that 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 Part 4: Environmental Factors of Heat-Related Illness, for the WBGT in the chart below.

       Table 8-1 Recommendations for Fluid Replacement During Warm Weather Conditions
    Easy Work Moderate Work   Hard Work
(Index F˚)
Water Intake
(qt. per hour)
Water Intake
(qt. per hour)
Water Intake
(qt. per hour
 78-81.9 Unlimited 0.5 Unlimited 0.75 40 work / 20 rest 0.75
82-84.9  Unlimited 0.5 50 work / 10 rest 0.75 30 work / 30 rest 1.0
85-87.9 Unlimited 0.75 40 work / 20 rest  0.75  30 work / 30 rest  1.0
88-89.9 Unlimited 0.75 30 work / 30 rest 0.75 20 work / 40 rest 1.0
90+ 50 work / 10 rest  1.0  20 work / 30 rest  1.0  10 work / 50 rest  1.0

* Fluid needs can vary based on individual differences (+- 0.25 qt. per hour) and exposure to full sun or full shade. Fluid intake should not exceed 1.5 qt. per hour daily fluid intake generally should not exceed 12 qts. This is not to suggest limiting fluid intake by highly conditioned persons, who may require greater than 12 qts. daily. 
Note: Rest = sitting or standing in the shade if possible. Adapted from DOD [2007].

For example - On a warm day in Rochester, NY the NWS Heat Index comes out to 95°F.  Imagine a gardener out in the sun tending their garden, they need to add 10°F 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, the glasses you have around your house are pint glasses or slightly smaller. Since 2 pts = 1 qt., 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 when 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 98.6°F. Anytime we are doing extra work our muscles increase heat production substantially. That heat must be dissipated to keep us 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.

    Table 6-2 Work/Rest Schedules for Workers Wearing Normal Work Clothing. No visible shadows from the sun, working in full shade or at night
 Adjusted Temperatures Light Work Moderate Work Heavy Work
90  Normal  Normal  Normal
91  Normal  Normal  Normal
92  Normal  Normal  Normal
93 Normal   Normal  Normal
94  Normal  Normal  Normal
95  Normal  Normal  45 work / 15 rest
96  Normal  Normal  45 work / 15 rest
97 Normal Normal 40 work / 20 rest
98  Normal Normal 35 work / 25 rest
99 Normal Normal 35 work / 25 rest
100 Normal 45 work / 15 rest 30 work / 30 rest
101 Normal 40 work / 20 rest 30 work / 30 rest
102 Normal 35 work / 25 rest 25 work / 35 rest
103 Normal 30 work / 30 rest 20 work / 40 rest
104 Normal 30 work / 30 rest 20 work / 40 rest
105 Normal 25 work / 35 rest 15 work / 45 rest
106 45 work / 15 rest 20 work / 40 rest Caution*
107 40 work / 20 rest 15 work / 45 rest Caution*
108 35 work / 25 rest Caution* Caution*
109 30 work / 30 rest Caution* Caution*
110 15 work / 45 rest Caution* Caution*
111 Caution* Caution* Caution*
112 Caution* Caution* Caution*

With the assumption that workers are physically fit, well-rested, fully hydrated, under age 40, have adequate water intake, and that there is 30% RH and natural ventilation with perceptible air movement. 

* Note: Adjust the temperature reading as follows before going to the temperature column in the table. 
Full sun (no clouds): add 13˚
Partly cloudy/overcast: Add 7˚
No shadows visible/ work in the shade or at night: No adjustment
Per relative humidity:
10% Subtract 8˚
20% Subtract 4˚
30% No adjustment 
40% Add 3˚
50% Add 6˚
60% Add 9˚

* Caution - For high levels of heat stress consider rescheduling activities


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 of what sorts of activities fall into which categories. We have distilled their cumbersome charts into something more palatable:

Workload / Activity Level  Activity Examples
 Rest Sitting, thinking
Light Sewing, writing, drawing, driving a car, a slow walk, standing
 Moderate Continuous normal walking, picking fruit and veggies, hammering nails, cooking, raking, painting
Heavy  Carrying loads, shoveling, landscaping, fast walking (>4mph)
Very Heavy Any activity is done at or near the maximum pace for example 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 86°F and because he lives in Rochester, NY, he can assume the humidity will be 70%. This puts the heat index at 95°F. To understand your heat index see blog Part 4: Environmental Factors of Heat-Related Illness.

Since his veggies need the sun to thrive he can assume that he will be in direct sunlight so that adds another 10°F degrees. This puts the environment at 105°F. 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 for him 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 sleeves or short sleeves 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).

   Table 3-2 Clothing adjustments factors for types of clothing
  Clothing adjustment factors (˚C-WBGT)
Clothing  Previous 2006
Work clothing (baseline) 0 0
Cloth overalls 3.5 0
Double-layer cloth clothing 5 3
Spunbound melt-blown synthetic (SMS) Coveralls - 0.5
Polyolefin coveralls - 1
Limited-use vapor-barrier coveralls - 11

Adapted from Bernard TE, Threshold Limit Values for Physical Agents Committee, ACGIH [2014].


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 and can cause sunburn. A good example of 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 desert 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 that protect their skin from harmful UV rays and allow for maximum airflow. Even if it appears counterintuitive to a Western audience. 

A Bedouin man wears 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, or umbrellas. Please note that 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 your 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 in 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 airflow - fans, clothing

  4. Reducing solar radiation - umbrellas, clothing

Each of these techniques has 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 custom cooling plan.


   READ PART 6        

   Visit #HeatIQ Home      



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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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