What is Heat Index?
When there’s much moisture in the air, the body can’t cool down as quickly by sweating. Even though the thermometer says one thing, the human body actually feels hotter. This feeling of extra heat is called the “heat index.”
The heat index (HI), also known as the apparent temperature, measures how hot it feels to humans by combining the effects of temperature and humidity. Unlike standard temperature readings, which only reflect air temperature, this index accurately represents how the human body perceives heat.
Heat Index vs Temperature
Temperature readings can be misleading, as high humidity levels can make it feel significantly hotter than the actual temperature, increasing the risk of heat stress. For example, a temperature of 90°F with 70% humidity has a HI of 105°F, making it feel much hotter than the actual temperature.
Why Understanding It Matters
The impact of heat on the human body can range from mild discomfort to severe health risks. Understanding HI is important for several key reasons:
Accurate Perception of Heat
When you understand HI, you can better plan for safety measures to protect yourself and your workers. Here’s a table of the four risk levels from the National Weather Service (NWS) and their effects on health and workplace productivity to give you a clearer idea of the associated risks:
| Risk Level | Heat Index Range | Effects on Humans | Effects on Workplace Productivity |
| Lower (Caution) | Less than 91°F (32°C) | Mild discomfort, minimal impact | Minimal impact on productivity, slight discomfort |
| Moderate | 91°F to 103°F (32°C to 39°C) | Increased sweating, fatigue, heat cramps | Reduced productivity due to fatigue and discomfort |
| High | 103°F to 115°F (39°C to 46°C) | Heavy sweating, dehydration, risk of heat exhaustion | Significant reduction in productivity, increased errors made at work |
| Very High to Extreme | Greater than 115°F (46°C) | Severe dehydration, heat stroke, life-threatening conditions | Severe impact on productivity, inability to work effectively |
Health Impact Assessment
Measuring the apparent temperature is crucial for assessing the potential health risks associated with hot weather, as it can help you determine whether working in certain conditions is safe. It’s essential to take precautions to prevent heat stress and other heat-related illnesses.
Workplace Safety
Understanding HI levels is crucial for employers and workers to assess work safety. This is especially important in industries like construction, agriculture, and landscaping that involve outdoor work or non-air-conditioned settings. High HI levels can also affect indoor work environments, such as factories or warehouses lacking air conditioning. Awareness of the heat level enables informed safety decisions and the implementation of necessary precautions to ensure everyone’s well-being.
Emergency Preparedness
Emergency response and preparedness organizations depend on HI data to predict and address heat-related crises. Awareness of heat levels enables these entities to prepare themselves for possible medical emergencies linked to heat exposure.
Adapting to Climate Change
If global warming hits 1.5°C above pre-industrial levels, extreme heat could cause up to 10% of deaths during heat waves. The 1.5°C target, set by the Paris Agreement, is crucial, as exceeding this threshold could lead to severe impacts. Understanding the heat levels can help us prepare and adapt to the effects of climate change.
How to Calculate Heat Index
There are methods to calculate the HI, such as:
- Method 1: Using the Heat Index Equation: This formula is complex, considering both air temperature and humidity, and involves several variables and constants that can be hard to calculate manually.
- Method 2: Using the Chart: The US National Weather Service (NWS) has created a chart illustrating the HI values corresponding to various combinations of air temperature and relative humidity.
- Method 3: Using Online Calculators: You input the temperature and relative humidity, and the tool will give you the accurate index level. These tools are readily accessible and can be used for free.
OSHA Guidelines
The US Occupational Safety and Health Administration (OSHA) currently doesn’t have a specific standard for addressing heat-related hazards. However, OSHA does provide guidelines and resources to help manage the risks associated with high HI values.
HI Categories
As the previous table above shows, OSHA has established four categories, each with corresponding recommended action. These are:
- Less than 91°F (32°C): Basic heat safety and planning measures are required.
- 91°F to 103°F (32°C to 39°C): Implement precautions and heighten awareness.
- 103°F to 115°F (39°C to 46°C): Additional precautions are needed to protect workers.
- Greater than 115°F): Workers and others outside in the field now need even more aggressive protective measures.
OSHA-NIOSH Heat Safety Tool App
This free mobile app provides real-time HI information, hourly forecasts, and risk levels based on location to prevent heat-related illnesses. It offers safety recommendations like hydration reminders and break schedules, and details symptoms of heat-related illnesses with first aid instructions.
National Emphasis Program (NEP)
OSHA has issued an Exposure to Outdoor and Indoor Heat-Related Hazards National Emphasis Program (NEP). It outlines policies to identify and mitigate worker exposures to heat-related illnesses and injuries in high-hazard industries, including general industry, construction, maritime, and agriculture.
Employer Responsibilities
Under Section 5(a)(1) of the OSH Act, OSHA mandates that employers ensure a safe work environment, which includes safeguarding workers from extreme heat. Some of the employer’s responsibilities include:
- Developing a heat hazard protection program
- Providing access to hydration and rest
- Allowing for acclimatization
- Implementing a buddy system
- Training workers and supervisors
- Establishing a communication system for reporting
Indoor and Outdoor Heat Illness Prevention Guidelines
OSHA also has specific guidelines for preventing heat illnesses, and part of this involves managing HI levels indoors and outdoors. However, in some states, these guidelines may be modified to cater to different cultures and environments.
An example of this is California’s Cal/OSHA’s specific guidelines on indoor and outdoor heat illness prevention. Indoor and outdoor workplaces have their own distinct regulations, as they have different working conditions.
For indoor work, there is the Heat Illness Prevention in Indoor Places of Employment regulation, which is used in managing heat levels and indexes in places such as restaurants, manufacturing facilities, warehouses, and other workplaces where temperatures can get high in a short amount of time. On the other hand, the Heat Illness Prevention for Outdoor Places of Employment regulation is for industries such as agriculture, construction, and landscaping, wherein workers typically feel a higher HI from being in hotter environments.
Monitoring Tools
Various tools are available to monitor HI levels to help organizations and businesses fulfill their responsibilities and ensure worker safety. Some of these include the following:
- Digital Thermometers and Hygrometers: Measures temperature and humidity, providing essential data for calculating HI.
- Wet Bulb Globe Temperature (WBGT) Meters: Offers a comprehensive indicator of heat stress by measuring temperature, humidity, wind speed, and solar radiation.
- Wearable Devices: Track personal exposure to heat, providing real-time data on body temperature and environmental conditions.
- Weather Stations: Continuously monitor local weather conditions, including temperature and humidity, with professional-grade accuracy.
- Monitoring Sensors: Devices that record data on temperature, humidity, and other environmental conditions and transmit it wirelessly to a central location for monitoring and analysis, like SafetyCulture’s. They can be:
- Temperature Sensors: to measure ambient temperature in work environments.
- Humidity Sensors: to track moisture levels in the air, critical for HI calculations.
- Air Pressure Sensors: to monitor atmospheric pressure, which can influence weather patterns and heat stress.
- Differential Air Pressure Sensors: to measure pressure differences between two points and help assess ventilation efficiency.
Explore SafetyCulture Monitoring Solution
Utilize advanced sensor technology to monitor assets, automate vital alerts, implement actions, and report urgent issues.
Talk to an expertBenefits of Using Sensors for Heat Index Monitoring
Sensors are just one of the tools you can use, but they can be the most helpful when managing heat risks. Some of their benefits include:
Early Intervention
Sensors can use real-time data to detect heat stress early and allow for quick intervention before it becomes a severe health issue. Sensors can continuously track vital signs like core body temperature and heart rate and trigger limits when crossing thresholds.
Accuracy and Reliability
Manual methods of calculating HI can be prone to human errors. In contrast, sensors provide 24/7 monitoring with precise and reliable data.
Portability and Affordability
New sensor technologies are small, lightweight, and inexpensive, making them easily portable and accessible. They can be attached to clothing or equipment without hindering worker movement and provide accurate data in real time.
Customizable Alerts
A wide range of sensor types (temperature, humidity, air pressure, differential air pressure) allows for tailored solutions that meet specific environmental monitoring needs. Alerts can also be customized to trigger events like high HI levels or low ventilation efficiency.
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