Heat Illness & Cold Stress
Why Humidity Makes
Heat More Dangerous
Heat More Dangerous
OSHA Heat Illness Prevention · NIOSH WBGT Guidelines · NYC Summer Humidity Index
New York City has one of the most oppressive summer humidity profiles of any major US city. In July and August, relative humidity regularly exceeds 80% at the same time temperatures top 90°F. This combination — not just temperature alone — is what makes NYC summer construction so dangerous. Understanding the heat index is required knowledge for every worker on this site.
Why Humidity Multiplies the Risk
- The body cools itself primarily through sweat evaporation. When relative humidity is high, sweat does not evaporate — it just drips off, providing no cooling
- At 95°F and 60% relative humidity: heat index (apparent temperature) = 113°F. The body is working as if it's 113°F
- At 90°F and 90% humidity: heat index = 122°F — dangerous conditions with prolonged exposure
- Workers performing heavy labor in these conditions can lose 2 liters of sweat per hour — replacement is critical
High-Risk Environments on NYC Sites
- Below-grade work (excavations, basements, mechanical rooms): hot, humid, minimal air movement — conditions amplify rapidly
- Working on black membrane or tar roofs: surface temperatures can reach 140°F+ in direct sun, creating a radiant heat burden on top of air temperature
- Enclosed spaces with forming and fresh concrete: concrete hydration releases heat and moisture simultaneously
NYC DOH Heat Emergency Protocol
- When NYC declares a Heat Emergency, OSHA's General Duty Clause obligates employers to take additional steps beyond routine heat illness prevention
- Additional water stations must be added; break frequency must increase; outdoor heavy work should be rescheduled to early morning hours when possible
- Workers should check weather.gov's "Excessive Heat" outlook daily during July–August
Discussion Questions
- It's 92°F with 75% humidity today. What is the approximate heat index, and what risk category does that represent?
- Why does a worker sweating heavily in a basement during concrete work need MORE water than a worker outside in the breeze?
- What specific environments on this site create the worst heat-humidity combination, and how can those conditions be mitigated?
- What additional steps must be taken on this site when NYC issues a Heat Emergency?
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