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Heat Stress: Work/Rest Schedules

Keywords:
Using work/rest schedules can decrease the risk of heat illness.

Things you need to know:

  • Continuous work in the heat is not advisable—you must take rest breaks to allow your body to cool down periodically.
  • Depending on environmental conditions and work intensity, you will need different amounts of rest in between your work cycles to prevent heat illness.
  • Wearing personal protective equipment (PPE) or wearing heavy, layered, or waterproof clothing can increase the risk of heat illness, even at mild temperatures.

 

Sample Work/Rest Schedule for Workers Wearing Normal Clothing*

The NIOSH work/rest table was originally developed for use in agriculture, and may be more applicable to surface mines than underground mines due to the high humidity levels in some underground mines.

Text box pointing to table row for 104 degrees. Text in box: Example. A worker performing heavy work in 104 degree temperatures should work for 20 minutes and rest for 40 minutes.

Temperature
(°F)
Light
Minutes Work/Rest
Moderate Minutes Work/Rest
Heavy
Minutes Work/Rest

90

Normal

Normal

Normal

91

Normal

Normal

Normal

92

Normal

Normal

Normal

93

Normal

Normal

Normal

94

Normal

Normal

Normal

95

Normal

Normal

45/15

96

Normal

Normal

45/15

97

Normal

Normal

40/20

98

Normal

Normal

35/25

99

Normal

Normal

35/25

100

Normal

45/15

30/30

101

Normal

40/20

30/30

102

Normal

35/25

25/35

103

Normal

30/30

20/40

104

Normal

30/30

20/40

105

Normal

25/35

15/45

106

45/15

20/40

Caution

107

40/20

15/45

Caution

108

35/25

Caution

Caution

109

30/30

Caution

Caution

110

15/45

Caution

Caution

111

Caution

Caution

Caution

112

Caution

Caution

Caution

Text box pointing to table row for 108 degrees. Text in box: Example. A worker performing moderate work in 108 degree temperatures should use extreme caution! It is possible to suffer heat exhaustion in less than 15 minutes under these conditions.

From NIOSH Criteria for a Recommended Standard, Occupational Exposure to Heat and Hot Environments, https://www.cdc.gov/niosh/docs/2016-106/pdfs/2016-106.pdf.

The NIOSH work/rest schedule is based on air temperature, with adjustment factors for sunlight and humidity. Other work/rest schedules are available, some of which are based on Wet Bulb Globe Temperature. In some situations, the NIOSH work/rest schedule may appear more restrictive than other work/rest schedules. Keep in mind the NIOSH table is only a guideline, meant to serve as a starting point for scheduling work in the heat. Whichever work/rest schedule is used, workers should ALWAYS pay attention to signs and symptoms of heat illness. Stop work, cool off, and drink fluids if you feel lightheaded, dizzy, ill, or if you experience muscle cramps or headache while working in the heat.

 

Temperature Adjustments for this Work/Rest Schedule

Adjust the temperature chosen in the left column based on:

Environmental Conditions
AND 
Humidity
  • Full sun (no clouds): Add 13 °F
  • Partly cloudy/overcast: Add 7 °F
  • No shadows visible, in the shade, or at night: No adjustment
  • 40% humidity: Add 3 °F
  • 50% humidity: Add 6 °F
  • 60% humidity or more: Add 9 °F
Example Adjustment

thermometer held in the sunA surface miner is working at a mine that uses the work/rest schedule as part of a plan to prevent heat illness. Conditions at the mine include a temperature of 90 °F, partly cloudy skies, and 50% humidity. Before consulting the table the worker should adjust the outdoor temperature by adding 7 °F for environmental conditions and 6 °F for humidity, resulting in an adjusted temperature of 103 °F. The worker should use this adjusted temperature to plan a work/rest schedule.

 

Case Study: Use of Work/Rest Schedule

A crew was shoveling ore out from under the primary conveyor at a surface mine in Arizona in August. The high temperature that day was 113 °F. The crew was rotating in 10-minute shifts and hydrating between shifts. Co-workers noticed signs of heat illness in two employees, and they were transferred to the medical station for evaluation. From there they were sent to the hospital, where they were given IV saline and released home. Both employees recovered after rehydration at the hospital.

Lessons Learned

The use of a work/rest cycle, good hydration, and team monitoring helped prevent this from turning into a severe incident.


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