How work/rest schedules are calculated

The math behind every plan.

The limits

NIOSH (2016-106) gives two limits in °C WBGT, based on metabolic rate M in watts:

• Acclimatized workers, REL = 56.7 − 11.5·log₁₀(M)
• Unacclimatized workers, RAL = 59.9 − 14.1·log₁₀(M)

Worked example

Moderate work (300 W), acclimatized crew: REL = 56.7 − 11.5 × 2.477 ≈ 28.2 °C WBGT. Up to that WBGT, the crew can work the full hour. At 30 °C, the hourly average rate has to come down to stay within the limit. With rest at 115 W, that's 30 minutes of work and 30 of rest per hour.

The ceiling

At rest (115 W), the REL is about 33.0 °C and the RAL about 30.8 °C. Above those values, even resting outdoors exceeds the limit, so the plan says to stop non-essential work.

Limits of the method

Clothing (coveralls, PPE) and radiant heat from roofs or machinery raise heat stress beyond the forecast. People differ, too. Use the plan as a starting point, and watch the crew.