How to Use a Heat Index Chart to Prevent Heat Disorders

Using a heat index is an accurate description of what the weather feels like outside. When it is very hot and humidity is high, knowing the heat index can help businesses protect employees from heat disorders.

What Is the Heat Index?

The National Weather Service defines the heat index as what the temperature feels like to an individual who is standing outside. It combines both the temperature and the amount of humidity in the air, which often means that the number is higher than the temperature, depending on the amount of humidity. 

For example, if the temperature outside is 90 degrees Fahrenheit, but the humidity makes it feel more like 94 degrees, then the heat index is 94 degrees. 

How Is the Heat Index Calculated?

A formula is used that takes into consideration both the temperature and the humidity level. In some cases, if the humidity level is low, it can lower what the temperature feels like. 

For example, if the temperature is 90 degrees and the relative humidity is 10%, then the heat index is 85 degrees. However, if the temperature is 90 degrees and the relative humidity is 50%, then the heat index is 95 degrees. 

If you’d like to calculate the heat index, this calculator is useful. You can also use this chart to get a rough estimate of the heat index based on current data. 

Why Does the Heat Index Matter? 

Simply knowing that it is 80 degrees outside is not enough information to accurately protect employees from heat disorders. The heat index provides more accurate intel about how to dress, how often breaks will be necessary, and how much water and other cooling supplies should be made available to employees working outside.

What Can Happen if Employees Are Exposed to a High Heat Index While Working? 

There are several heat disorders that can manifest quickly when the sun is blazing down and employees are working outside. These are the three most common disorders:

  1. Heat cramps: Heat cramps are muscle spasms and cramps that occur after sweating in intense heat or during heavy exercise. They are painful, but they are the mildest form of heat disorder and usually pass with rest, sports drinks, shade and time. 

  2. Heat exhaustion: When the body loses fluids and salt to sweat, and they are not replenished, the body is unable to cool itself, and heat exhaustion can occur. If this is untreated, it can turn into heat stroke.  Providing shade, rest and sports drinks can help. It may also be a good idea to remove excess clothing. If the condition does not get better or the person cannot take in fluids, an IV at the emergency room may be required. 

  3. Heat stroke: This is a life-threatening issue that occurs when the body is overwhelmed by heat. The first step is the same as for heat exhaustion but because medical attention is required immediately, an ambulance should be called. While waiting, follow the directions of the dispatcher, which will usually include removing excess clothing and potentially placing ice packs in the armpits if the person is unable to drink fluids. 

What Are the Signs of Developing Heat Issues?

Noticing things like the heat index and considering the level of physical activity to be performed and the required personal protective equipment that must be worn is a good place to start. If the heat index is high and the job requires employees to be outdoors for long periods, heavy PPE is required. It is important to provide ways to help employees manage heat conditions.

Additionally, if an employee has other issues to consider, such as chronic illness, obesity or respiratory issues, they may be at greater risk for all heat issues. If employees are sweating heavily, having a hard time breathing, slowing down or otherwise showing signs of intense heat, it’s time to give them a break and allow them to cool down before returning to work. 

How Often Should Breaks Be Provided?

The heat index and the labor being done should be factored in when coming up with a safe and healthy work-to-rest ratio each hour. 

Examples of light work include performing inspections, operating equipment, using simple hand tools or walking on flat ground. Moderate work includes carrying items that weigh between 20 and 40 pounds, using hand tools that require effort to use for less than 20 minutes and loading explosives. Heavy work means climbing, carrying equipment that weighs 40 pounds or more and using hand tools that require effort for more than 20 minutes. 

Light Work 

If the work is light, it may be okay to have people work for 50 minutes with a 10-minute break, all the way up to 105 degrees. This is the case if there is very light or no PPE needed and employees are in good health and have access to water, sunscreen, a hat and eye protection like sunglasses. 

However, at 106 degrees, the recommended schedule shifts to 45 minutes of work and 15 minutes of rest. At 107 degrees, it becomes 40 minutes of work and 20 minutes of rest. Each degree higher takes 5 more minutes away from work time and places it in the rest category. This is the case up until 110 degrees, at which point it is recommended that employees work no more than 15 minutes and rest for 45 minutes. After that, any work should be done with extreme caution. 

Moderate Work

If the work is moderate, the break schedule shifts quite a bit. A normal schedule of 50 minutes on and 10 minutes off continues up through the 99-degree mark. After that, 5 minutes per hour is taken from work time and added to rest time. This continues until 107 degrees, at which point the recommended work time is only 15 minutes with 45 minutes of break. For 108 degrees and higher, moderate outdoor work is not recommended at all. 

Heavy Work

If outdoor work requires heavy exertion, the normal 50 minutes on and 10 minutes off is acceptable up to 94 degrees. At 95 and 96 degrees, it becomes 45 minutes of work and 15 minutes off, shifting to an equal split of 30 minutes on and 30 minutes off by 100 to 101 degrees. Continue to move 5 minutes from work to rest time until 105 degrees is reached. At this point, 15 minutes of work with a 45-minute break is recommended. At any higher temperature, heavy work outdoors is not recommended. 

Use Caution

Remember that those temperatures should take the heat index into account. If the temperature is 100 degrees but the heat index is 107, rather than offering a normal work schedule of 50 minutes on and 10 minutes off for light work, employers should enforce a schedule of 40 minutes on and 20 minutes off for the employees’ protection. 

This chart from the Centers for Disease Control and Prevention can help you implement a safe work and break schedule for employees. 

Alsco Can Help

If you need to provide uniforms that are appropriate for hot weather, Alsco can help. We offer a variety of uniform options that provide some relief from the heat while providing the protection your employees need.

Not only do we help with uniform cleaning and maintenance, but we can also provide cleaning towels and other cleaning supplies that may be used outdoors. Contact Alsco today to learn more about how we can streamline your maintenance needs so that management can stay focused on other important aspects of the business.


What Is the Heat Index? National Weather Service.

Heat Index Calculator. National Weather Service.

Heat Index Chart With Health Effects and Safety Recommendations. Arizona Department of Health Services.

Heat-Related Illnesses (Heat Cramps, Heat Exhaustion, Heat Stroke). John Hopkins Medicine.

Heat Hazard Recognition. United States Department of Labor.

Heat Stress Work/Rest Schedules. National Institute for Occupational Safety and Health.

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