Protecting Outdoor Workers from Extreme Heat,
HeatSafe is here.

According to the National Weather Service (NWS), heat index risks are categorized into four levels based on apparent temperature (what it feels like when combining air temperature and relative humidity):

• Caution (80°F - 90°F): Fatigue is possible with prolonged exposure and/or physical activity.
• Extreme Caution (90°F - 103°F): Heat stroke, heat cramps, or heat exhaustion are possible with prolonged exposure and/or physical activity.
• Danger (103°F - 124°F): Heat cramps or heat exhaustion are likely; heat stroke is possible with prolonged exposure and/or physical activity.
• Extreme Danger (125°F or higher): Heat stroke is highly likely.

NWS Official Guideline: NWS Heat Index values are devised for shady locations. Exposure to full sunshine can increase heat index values by up to 15°F. HeatSafe includes a "Sunny / Shade" toggle to automatically apply this official correction factor for accurate field measurements.

OSHA's "Water. Rest. Shade." Campaign: Under the General Duty Clause, OSHA mandates that employers protect workers from extreme heat. The three core elements of prevention are:

• Water: Encourage workers to drink plenty of fluids (cool water) even if they do not feel thirsty (about 4 cups or 1 liter per hour during high heat).
• Rest: Implement recommended rest breaks to allow the body to cool down.
• Shade: Provide a fully shaded or air-conditioned rest area for recovery.

WBGT accounts for air temperature, humidity, wind speed, and solar radiation — four factors that determine how effectively the human body can cool itself through sweating. A plain air temperature of 95°F (35°C) may feel far more dangerous at 80% humidity than at 20% humidity. WBGT captures this real physiological heat burden.

Source: ISO 7243:2017 — Ergonomics of the thermal environment.

The US National Institute for Occupational Safety and Health (NIOSH) recommends the following work-rest ratios for outdoor workers under heat stress (Criteria Document for Occupational Exposure to Heat and Hot Environments, 2016):

• Light work (e.g., monitoring): 75% work / 25% rest per hour at WBGT ≥ 30°C (86°F).
• Moderate work (e.g., walking, carrying): 50% work / 50% rest per hour at WBGT ≥ 28°C (82°F).
• Heavy work (e.g., heavy lifting, digging): 25% work / 75% rest per hour at WBGT ≥ 27°C (81°F).

Source: NIOSH Publication No. 2016-106.

Under the General Duty Clause (Section 5(a)(1)) of the OSH Act, employers are required to provide a workplace free from recognized hazards, including extreme heat. OSHA's National Emphasis Program (NEP) on Outdoor and Indoor Heat-Related Hazards (Directive CPL 03-00-024, 2023) formally targets heat illness prevention across all industries.

Key employer obligations include: providing cool water near the work area, scheduling rest breaks in shade or air-conditioned environments, acclimatizing new workers, and training all workers on heat illness symptoms.

Source: OSHA.gov — Heat Illness Prevention Campaign.