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Temperature Stress

Heat Index Chart

9/1/21
Grainger Editorial Staff

How to use a heat index chart: Locate today’s predicted high temperature across the top column (Air Temp.). Down the left side (Relative Humidity), locate today’s predicted humidity. Follow across and down to find the “Apparent Temperature”. Keep in mind, this heat index chart was devised for shady, light wind conditions, so exposure to full sun can increase values by up to 15°F. Strong winds, particularly with hot, dry air can be extremely hazardous.

Air
Temp.
70°F
21°C
75°F
24°C
80°F
26°C
85°F
29°C
90°F
32°C
95°F
35°C
100°F
38°C
105°F
41°C
110°F
43°C
Relative
Humidity
Apparent Temperature (What it Feels Like)
(Degrees Fahrenheit/Degrees Celsius)
0% 64°/18° 69°/21° 73°/23° 78°/25° 83°/28° 87°/30° 91°/33° 95°/35° 99°/37°
10% 65°/18° 70°/21° 75°/24° 80°/26° 85°/29° 90°/32° 95°/35° 100°/38° 105°/41°
20% 66°/18° 72°/22° 77°/25° 82°/27° 87°/30° 93°/34° 99°/37° 105°/41° 112°/44°
30% 67°/19° 73°/23° 78°/25° 84°/28° 90°/32° 96°/35° 104°/40° 113°/45° 123°/51°
40% 68°/20° 74°/23° 79°/26° 86°/29° 93°/34° 101°/38° 110°/43° 122°/50° 137°/58°
50% 69°/21° 75°/24° 81°/27° 88°/31° 96°/35° 107°/42° 120°/49° 135°/57° 150°/66°
60% 70°/21° 76°/24° 82°/27° 90°/32° 100°/38° 114°/45° 132°/56° 149°/65°  
70% 70°/21° 77°/25° 85°/29° 93°/34° 106°/41° 124°/51° 144°/62°    
80% 71°/22° 78°/25° 86°/29° 97°/36° 113°/45° 136°/58° 157°/69°    
90% 71°/22° 79°/26° 88°/31° 102°/39° 122°/50° 150°/66° 170°/77°    
100% 72°/22° 80°/26° 91°/33° 108°/42° 133°/56° 166°/74°      

Likelihood of heat disorders with prolonged exposure or strenuous activity:

 
Heat Index Risk Level Protective Measures
Less than 91°F (32°C) Lower (caution) Basic heat safety and planning
91° to 103°F (32° to 39°C) Moderate Implement precautions and heighten awareness
103° to 115°F (39° to 46°C) High Additional precautions to protect workers
Greater than 115°F (46°C) Very high to extreme Triggers even more aggressive protective measures

Source: National Oceanic & Atmospheric Association & OSHA

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The information contained in this article is intended for general information purposes only and is based on information available as of the initial date of publication. No representation is made that the information or references are complete or remain current. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.

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