State heat-stress rules are spreading, and OSHA's proposed federal rule is moving forward. A meter on the wall is not a monitoring program. This guide covers how to set up indoor monitoring that protects your workers and produces records you can stand behind, and where the HIX1800 fits.

If your warehouse, kitchen or production floor gets hot, you already know it. What regulators increasingly want to know is whether you can show you were watching: the right instrument, in the right place, with records to back it up. Indoor heat index meters have become the everyday tool for that job, but a well-planned setup and a box that produces meaningless numbers are far apart.

What you're measuring

The heat index, sometimes called the "feels-like" or apparent temperature, combines air temperature and relative humidity to show how hot conditions feel. Humid air feels hotter because it slows the evaporation of sweat, the body's main way of cooling. Indoor heat index meters calculate it with the National Weather Service's Rothfusz regression, the standard equation based on Steadman's heat-stress research.

Getting that calculation right matters. The main equation isn't valid in all conditions: a simpler formula applies below about 80°F, and extra corrections apply at very low and very high humidity. A device that uses only the main formula can read several degrees off in exactly the range where the 80°F action level sits. Humidity makes a real difference: 83°F at 70% relative humidity feels like about 88°F, enough to cross a threshold a plain thermometer would never flag.

What the rules expect

OSHA's proposed indoor and outdoor heat rule sets two action levels: an initial heat trigger at a heat index of 80°F and a high heat trigger at 90°F. These line up with the heat index bands used by the National Weather Service and OSHA:

Several states already enforce heat standards regardless of the federal proposal. Oregon and Maryland act at a heat index of 80°F, with added protections above 90°F. Minnesota regulates indoor work using WBGT limits tied to how strenuous the work is, and California's indoor heat standard is in effect. Even where no specific standard applies, OSHA can cite employers for heat hazards under the General Duty Clause.

The proposed federal rule also describes what an adequate program looks like: identify each work area that could reasonably reach the thresholds, have a monitoring plan for each one, and keep records of on-site measurements for six months. One meter at the front of a large building, with no records, doesn't meet that bar.

Setting up the system, step by step

1. Map your hot zones

Walk the facility and identify every area that could reasonably reach the heat thresholds: packing lines, mezzanines, areas near ovens or shrink-wrap tunnels, enclosed rooms with poor airflow. One meter rarely represents a whole building. Plan one monitor per distinct work zone.

2. Mount monitors where people work

Place each monitor at about chest height for the workers in that area, and give readings time to settle. Heat rises, so a sensor near the top of a 20-foot wall can read several degrees off from conditions on the floor. Put it near the hot spots rather than the cool corner by the office, and keep it out of the direct path of fans, vents, open dock doors and sunlight.

3. Set alarms to the standard you're under

Program alarm setpoints to match the rule that applies to you, commonly the 80°F and 90°F triggers or your state's levels. An inspector can reasonably ask about this, so the device's alarm settings should match the rule on paper.

4. Make the alarm hard to miss

A large, bright display lets workers and supervisors check conditions without stopping work, and an audible alarm is better than relying on someone to look. In large spaces, connect an external strobe, siren or bell so the warning reaches everyone.

5. Keep records

Regular records of temperature, humidity and heat index for each shift are what show compliance. With a six-month records requirement proposed, a monitor with no record trail leaves you unable to show past conditions, even if they were fine. Decide up front how you'll capture and keep the readings.

6. Calibrate, and keep calibrating

Readings are only as good as the calibration. Start with a calibrated unit, add a NIST-traceable certificate where you need formal documentation, and schedule recurring calibration checks: yearly in clean areas, more often in dusty or solvent-heavy ones. A meter that quietly reads low is a safety problem, not just a paperwork one.

The HIX1800 Heat Index Monitor

The HIX1800 is a wall-mounted indoor monitor that measures temperature, relative humidity and heat index together, and shows the heat index on large red LED digits you can read from across the floor.

View the HIX1800 or add NIST-traceable calibration.

How the HIX1800 covers the checklist

Why indoor heat deserves attention

Outdoor work accounts for most heat-related deaths. Agriculture, construction and similar work made up roughly two-thirds of heat-related occupational deaths from 2000 to 2010, and construction alone accounted for 36% between 1992 and 2016. But indoor exposure is far from minor. A review of 66 OSHA heat enforcement cases from 2011 to 2016 found heat-related injuries and illnesses, including deaths, in both outdoor (34 cases) and indoor (29 cases) settings, close to an even split. Under-reporting likely hides more.

In a hot warehouse, kitchen or plant without adequate climate control, a properly placed, calibrated meter with good records is solid evidence of an active monitoring program. The instrument is the easy part. Choosing the right zones, mounting height and thresholds, keeping real records and calibrating regularly is what protects your people.

This article is general information for facility and safety planning, not legal advice. Heat-stress requirements vary by location and change over time, so confirm which standard applies to your facility. The HIX1800 is designed for indoor use out of direct sunlight and does not replace WBGT measurement in high-radiant settings.

Sources

1. OSHA, Heat Injury and Illness Prevention in Outdoor and Indoor Work Settings, SBREFA background document (Aug. 21, 2023) and proposed rule (Federal Register, Aug. 30, 2024).
2. OSHA, Heat Hazard Recognition, and Temporary Worker Initiative Bulletin No. 12.
3. National Weather Service / NOAA Weather Prediction Center, The Heat Index Equation.
4. Minnesota Department of Administration, Heat Stress Guide.
5. AlertMedia, New OSHA Heat Regulations (2025).
6. Tustin et al. (2018), MMWR; Gubernot et al. (2015), Am. J. Ind. Med.; Center for American Progress (2024).