Why Temperature Alone Doesn't Tell the Whole Story
Temperature alone won't tell you how dangerous it is outside. A 95-degree day with high humidity can be far more dangerous than 100 degrees with low humidity. The heat index (sometimes called the "apparent temperature" or "feels like" temperature) combines air temperature and humidity to show how hot conditions actually feel to your body. Knowing the heat index action levels helps you make smart decisions about when and how to work safely.
How the Heat Index Works
What Heat Index Measures
High humidity prevents sweat evaporation, which means your body can't cool itself as effectively. Risk increases even at moderate temperatures when humidity is high.
Why Humidity Matters
Sweat must evaporate to carry away heat. High humidity prevents that evaporation, and heat builds up in your body. To put it in perspective: 90°F with 40% humidity gives you a heat index of about 91°F, while 90°F with 80% humidity pushes the heat index to 112°F. Same temperature, dramatically different risk.
Heat Index Risk Categories
OSHA/NIOSH Framework
The National Institute for Occupational Safety and Health (NIOSH) and OSHA have established heat index action levels:
| Heat Index | Risk Level | Color |
|---|---|---|
| Below 91°F | Lower (but not zero) | Yellow |
| 91-103°F | Moderate | Orange |
| 103-115°F | High | Red |
| Above 115°F | Very High/Extreme | Maroon |
Action Levels in Detail
Lower Risk: Heat Index Below 91°F
Allow workers to take breaks as needed and remind them of heat illness symptoms. Watch for early signs in new or returning workers. Standard precautions apply, and this is a good time for heavier tasks.
Moderate Risk: Heat Index 91-103°F
Establish scheduled rest breaks in shade, monitor workers closely, limit heavy work in direct sun, and provide an acclimatization period for new workers. Schedule heavy work for cooler hours, increase break frequency, and consider shortening the overall work day.
High Risk: Heat Index 103-115°F
Water must be constantly available and actively pushed. All rest breaks should be in shade with active cooling measures in place. Monitor all workers closely and respond immediately to any heat illness symptoms. Limit work to light tasks during peak heat, extend midday breaks, use a mandatory buddy system, and consider stopping non-essential outdoor work entirely.
Extreme Risk: Heat Index Above 115°F
If work must occur, engineering controls and constant monitoring are required. Mandate shade and rest at least every 45 minutes, push water consumption aggressively, and provide active cooling for all workers. Have an emergency plan in place. Extend cooling breaks, reduce work pace, bring in additional workers if the task is essential, and consider shifting necessary work to nighttime.
Quick Reference Chart
Heat Index Action Summary
| Heat Index | Key Actions | Work Level |
|---|---|---|
| <91°F | Water, shade available | Normal |
| 91-103°F | More water, scheduled breaks, monitor | Modified |
| 103-115°F | Mandatory breaks, active cooling, reduce intensity | Significantly reduced |
| >115°F | Stop most work, emergency protocols, essential only | Minimal/stopped |
Finding Current Heat Index
Sources
The National Weather Service issues heat advisories based on the heat index, available online, through apps, or on weather radio. Most weather apps display an hour-by-hour heat index, and you can set alerts for high levels. For remote operations, on-site measurement gives the most accurate reading, and wet bulb globe temperature (WBGT) is the gold standard.
When to Check
Check before starting heavy work to verify conditions, monitor throughout the day as conditions change, and confirm the environment when symptoms appear.
Applying Heat Index in Practice
Morning Planning
Check the forecasted heat index first thing, identify when peak heat index is expected, plan your work schedule accordingly, assign heavy tasks to lower heat index periods, and establish a break schedule based on expected conditions.
During the Day
Be ready to adjust if conditions are worse than expected, prepared to stop work if the heat index rises unexpectedly, and watching workers for heat illness signs throughout.
Decision Points
When deciding whether to continue working, consider what the heat index is expected to do over the next few hours, how intense the work is, whether workers are acclimatized, and what cooling resources are available.
Factors That Increase Risk Within Categories
Move to Higher Precautions If:
Work conditions are challenging: no shade available, PPE required (which adds heat load), working with hot equipment, or no AC available for breaks.
Individual worker factors also matter: workers returning from absence, older workers, workers on certain medications, and workers with health conditions all warrant extra caution.
Environmental factors play a role too, including lack of breeze or airflow, radiant heat from surfaces, and enclosed or poorly ventilated areas.
Texas-Specific Considerations
Typical Texas Summer Conditions
Many Texas summer days exceed the 103°F heat index threshold. July and August often reach extreme risk levels. Humidity varies significantly by region, which makes a big difference in how dangerous any given temperature actually is.
Regional Variation
| Region | Typical Humidity | Heat Index Impact |
|---|---|---|
| Gulf Coast | High | Dramatically elevated |
| Central Texas | Moderate-High | Significantly elevated |
| North Texas | Moderate | Moderately elevated |
| West Texas | Low | Close to actual temp |
| Panhandle | Low | Close to actual temp |
Planning for Texas
Normal summer operations in Texas require heat precautions as a baseline. Schedules should assume early morning work in summer, and heat illness prevention is really a year-round priority in this state.
Documentation and Communication
Daily Heat Assessment
Record the forecasted heat index, actual conditions, risk level, precautions implemented, and any incidents for each working day.
Communication to Workers
At the start of the day, communicate the risk level and what it means, the specific precautions for that day, and a reminder of heat illness symptoms. Throughout the day, send reminders about water and breaks, and check in on how workers are feeling.
Bottom Line
Heat index matters more than temperature alone for assessing risk. A 91°F heat index triggers enhanced precautions, a 103°F heat index requires significant work modifications, and anything above 115°F means stopping non-essential work. Check the heat index daily and throughout the day, because the same temperature can carry very different risk depending on humidity.
Texas summers routinely exceed moderate risk thresholds, so plan accordingly. When multiple risk factors are present (high heat index, no shade, heavy work, unacclimatized workers), bump your precaution level up a category. Document your decisions about heat exposure, and communicate clearly with workers about conditions and expectations. The numbers don't lie, and neither does a heat-related emergency.
Related Articles
- Heat Illness Prevention for Ranchers
- Recognizing Heat Exhaustion and Stroke
- Scheduling Work Around Heat
- First Aid for Heat Emergencies
Texas Resources
- National Weather Service: Local heat index forecasts, heat advisories
- OSHA: Heat illness prevention technical guidance
- NIOSH: Heat stress criteria and recommendations
- Weather apps: Real-time heat index monitoring
