The heat index is a "muggy heat" measure calculated from air temperature and relative humidity, and Taiwan OSHA's heat hazard risk Levels 1 to 4 are determined by it. WBGT adds the effect of solar radiation and wind, and is closer to the heat people really face on site. In strong sun or near a heat source, the gap between the two is greatest.
Many people cannot tell heat index, feels-like temperature and WBGT apart, and the numbers they find often do not match. This article explains what the heat index is, how to look up the heat index table and read the levels, compares the heat index with WBGT in a table, and finally tells you when the heat index is enough and when you must measure WBGT.
What Is the Heat Index?
The heat index combines air temperature and relative humidity to show how hot it feels. The higher the humidity, the less sweat evaporates, and the hotter the same air temperature feels. For example, at an air temperature of 32°C, the heat index in dry air and in muggy, humid air is very different.
The heat index assumes conditions of "shade and a light breeze". The US National Weather Service (NWS) explains that the heat index is worked out for shade and light wind, and in direct sunshine it can be up to about 15°F (about 8°C) higher. This is the heat index's biggest limitation: it does not know whether you are in the sun.
How Do You Read the Heat Index Table?
The OSHA Guidelines for the Prevention of Heat Hazards in High-Temperature Work give three steps:
- Get the temperature and relative humidity: look up the values at the work location or a nearby station from the Central Weather Administration, or measure on site.
- Look up the heat index table (Schedule 1): temperature is on the vertical axis and relative humidity on the horizontal axis, and the number where they cross is the heat index. If the table has no exact value, assess using the higher cell. The guidelines give this example: on site it is 32.0°C and 67% relative humidity, so use 32.2°C and 70%, which gives a heat index of 40.6.
- Check the heat hazard risk level table (Schedule 2): find Level 1 to 4 from the heat index (Level 3 in the example above) and take the management measures for that level.
If you do not want to look up the table yourself, go straight to the Ministry of Labor's High-Temperature Work Protection Information Website (hiosha.osha.gov.tw) for the hourly heat index and level for your area.
How Do You Read Heat Index Levels?
- Below the risk levels: the High-Temperature Work Protection Information Website states that a heat index below 26.7 is below the high-temperature risk levels.
- Levels 1 to 4: the higher the heat index, the higher the level, and each level has corresponding hazard prevention and management measures. The website states that a heat index above 58.9 is all Level 4.
- Raise by one level: for work in direct sunshine, or in non-breathable, heavy or impermeable protective clothing, raise the assessed level by one.
- Level 4: under Article 303-1 of the Regulations on Occupational Safety and Health Facilities, provide shade and cooling equipment, a cooling rest area nearby and sufficient drinking water.
For what to do at each level and the penalties, see Heat Hazard Regulations and Penalties.
How Is the Heat Index Calculated?
The heat index is calculated with a multi-term polynomial regression formula (the Rothfusz formula used by the NWS) from air temperature and relative humidity. It has many terms and is not suited to hand calculation. In practice, simply look it up in the heat index table in the guidelines or on the information website. Remember that the heat index uses only two numbers, temperature and humidity, so a thermo-hygrometer is all you need. That is both why it is convenient and why it is not enough.
How Does the Heat Index Differ from WBGT?
| Comparison Item | Heat Index | WBGT (Wet Bulb Globe Temperature) |
|---|---|---|
| What data it uses | Air temperature, relative humidity | Dry-bulb temperature, natural wet-bulb temperature, globe temperature (reflecting humidity, wind and radiant heat) |
| Does it show sunshine and radiant heat? | Cannot see (assumes shade) | Can see |
| Does it show the effect of wind? | Cannot see (assumes a light breeze) | Can see (through natural wet-bulb and globe) |
| How it is obtained | Looked up from weather station data, or the information website | Measured on site with a WBGT instrument |
| Where Taiwan regulations use it | Outdoor work heat hazard risk Levels 1 to 4 (guidelines, Article 303-1 of the Facilities Regulations) | Standard for Work and Rest Schedules for Workers in High-Temperature Work (rest proportion by work intensity) |
| Internationally | US NWS public warnings, some US state laws | ISO 7243, Japanese and Thai regulations, sports events and schools |
| Suitable | Checking regional risk and planning work before starting each day | Knowing how hot your own site, plant or sports field really is |
Note: heat index and WBGT values cannot be compared directly. A "32" as a heat index and a "32" as WBGT mean completely different levels of heat. WBGT numbers are usually much lower than heat index numbers, and the thresholds are different too.
When Is the Heat Index Alone Not Enough?
- In strong sun: roofs, steel frames, roads, sports fields. The heat index assumes shade, so it underestimates when in the sun.
- Net houses, greenhouses, metal-roofed buildings: the Ministry of Labor's measurement research found that measured values in farm net houses were clearly higher than the website's predictions.
- Near heat sources: machinery, furnaces, scorching ground and walls.
- Remote locations far from a weather station: when Academia Sinica introduces health meteorology, it also points out that the time and place of high temperatures do not match the time and place of high WBGT values, and the two cannot replace each other.
For these places, the Ministry of Labor recommends measuring WBGT on site when necessary. For how to calculate WBGT, see How to Calculate WBGT: Outdoor and Indoor Formulas.
Can You Use a Heat Index Meter?
A heat index meter is a thermo-hygrometer that converts to heat index automatically. Measuring temperature and humidity on site and looking up the level is more accurate than using a distant weather station, but it has no globe temperature, so it still cannot see sun exposure and radiant heat. To measure WBGT, choose an instrument with a globe temperature. For a comparison, see How to Choose a WBGT Meter.
How Should the Two Be Used Together?
- The day before and before work: look at the heat index and level on the information website to decide the day's working hours and basic measures.
- The hottest spots on site: use WBGT monitoring to find places and times hotter than the area value, and add rest and cooling.
- Keep records: the level, on-site values and measures taken, kept together, form a complete heat hazard management record.
Heat Safety Guardian is a heat hazard alert system developed in-house by YenProtek. It calculates the WBGT heat index in real time and, when limits are exceeded, triggers on-site sound and light alarms and LINE notifications. It measures air temperature, humidity, globe temperature, wind speed and wind direction together, and presents on-site conditions according to the heat hazard risk levels in the OSHA guidelines, with alert thresholds customisable to the site. See the Heat Safety Guardian product page, or go to Online Quote to see configurations.
Frequently Asked Questions
Q1: What does heat index mean?
The heat index is a measure of how muggy it feels, calculated from air temperature and relative humidity, showing how hot it feels to people. Taiwan's OSHA uses the heat index to determine heat hazard risk Levels 1 to 4 for outdoor work.
Q2: What heat index is dangerous?
The High-Temperature Work Protection Information Website states that a heat index below 26.7 is below the high-temperature risk levels. Above that, it is divided into Levels 1 to 4 by heat index, and anything above 58.9 is Level 4. Raise the level by one for sun exposure or non-breathable protective clothing.
Q3: Is the heat index the same as the "feels like" temperature?
The concepts are close and both describe how hot it feels, but weather agencies in different countries calculate them differently. For workplace heat hazards in Taiwan, use the heat index table in the OSHA guidelines; South Korean regulations use the feels-like temperature.
Q4: Which is more accurate, the heat index or WBGT?
They are used for different things. The heat index suits regional risk and daily work planning. WBGT adds solar radiation and wind, and is closer to the heat people really face on site. In strong sun, in net houses or near heat sources, WBGT reflects the actual situation better.
Q5: Is a heat index of 40 the same as a WBGT of 40?
No. They are calculated differently, so the numbers cannot be compared directly. WBGT numbers are usually lower than heat index numbers, and the thresholds are different too.
Further Reading: WBGT and Heat Hazard Series
- What Is WBGT? Wet Bulb Globe Temperature Explained (Series Overview)
- How to Calculate WBGT: Outdoor and Indoor Formulas with Examples
- WBGT Rest Break Schedule: High-Temperature Work Rest Standard Table
- Heat Hazard Regulations and Penalties: What Employers Must Do Under the 2026 Occupational Safety and Health Act
- How to Apply for Heat Hazard Subsidies
- How to Choose a WBGT Meter: Handheld vs Fixed Monitoring Station
- Managing Heat Hazards on Construction Sites: Heat Stroke Prevention Checklist
References
- Occupational Safety and Health Administration, Ministry of Labor, Guidelines for the Prevention of Heat Hazards in High-Temperature Work, third edition, Schedules 1 and 2.
- High-Temperature Work Protection Information Website (hiosha.osha.gov.tw).
- Regulations on Occupational Safety and Health Facilities, Article 303-1.
- Ministry of Labor, "Real-Time Measurement to Prevent Heat Hazards: Recommended Method for WBGT Measurement in High-Temperature Work", 2025-08-26.
- U.S. National Weather Service, Heat Index (explanation that the heat index assumes shade and light wind).
- Academia Sinica, Health Meteorology and WBGT explanation.
Article updated: 2026-11-10.
