Low-Cost WBGT Heat Stress Station with Work-Rest Alerts for Outdoor Workers

The problem: heat kills workers, and thermometers underestimate it

Summer heat in Pakistan, India and the Gulf regularly reaches levels that are dangerous for anyone doing physical work outdoors. Construction labourers, brick kiln workers, farm workers and traffic police work through it. Heat exhaustion and heat stroke are preventable with rest, water and shade at the right times, but supervisors need a clear signal of when "hot" becomes "dangerous".

Air temperature is a poor signal. Humid heat is worse than dry heat at the same temperature because sweat cannot evaporate. Direct sun adds a large heat load, and wind helps. Wet bulb globe temperature (WBGT) combines all of these and is the measure used in occupational heat standards such as ISO 7243. The problem is that proper WBGT meters are expensive handheld instruments, so most sites in South Asia never use one.

What already exists and the gap

• Professional heat stress meters, such as Kestrel heat stress trackers, measure WBGT accurately and cost far more than small contractors spend.
• OSHA's outdoor WBGT calculator estimates WBGT from weather data for US locations, using the Liljegren model.
• Weather apps report temperature and sometimes a heat index, which ignores sun and wind.
• Research prototypes of low-cost WBGT sensors exist, often without simple alerts for workers.

The gap is a cheap, robust station that a site supervisor can mount on a pole, which shows a coloured light everyone can see and sends a message when it is time to rest.

Inspiration

Liljegren and colleagues at Argonne National Laboratory published "Modeling the Wet Bulb Globe Temperature Using Standard Meteorological Measurements" (Journal of Occupational and Environmental Hygiene, 2008). Their model estimates WBGT from air temperature, humidity, wind speed and solar radiation with accuracy better than 1 °C in their tests, and it is the algorithm behind OSHA's WBGT calculator. Your project uses measured globe temperature where possible and the Liljegren model as a cross-check.

What you build

• The station measures temperature and humidity in a shaded, ventilated screen, globe temperature inside a black sphere in the sun, and wind speed.
• The firmware reads sensors every minute, smooths readings and computes WBGT.
• The alert logic maps WBGT to levels with recommended work and rest minutes per hour for the type of work.
• The supervisor sees the light change colour, receives a message and finds a daily report of heat exposure.

The technically interesting part

• The black globe. Standard globes are 150 millimetres across and respond slowly. A smaller globe responds faster but reads differently. Build one, document its response time and apply a correction, comparing against a reference if you can borrow one.
• Radiation shields. A humidity sensor in direct sun reads far too hot. Build or buy a ventilated shield and show the difference with and without it.
• Computing natural wet bulb temperature. Use the Liljegren equations, implemented in Python first and then on the microcontroller or the server.
• Alerts people trust. Avoid flicker between levels with hysteresis and averaging over several minutes. Write messages in plain Urdu and English.

How to evaluate it

• Accuracy: compare your WBGT against a reference heat stress meter, or against WBGT computed from a nearby professional weather station, over several days.
• Sensor checks: temperature and humidity against a calibrated reference indoors.
• Response time: how quickly the globe reading settles after moving from shade to sun.
• Usability: a week at a real site, with supervisor and worker feedback on the alerts.

Suggested plan

• Weeks 1 to 3: read about WBGT and heat stress guidance, choose sensors, build the radiation shield and globe.
• Weeks 4 to 6: firmware, sensor reading, MQTT logging.
• Weeks 7 to 9: WBGT calculation, Liljegren model, alert levels.
• Weeks 10 to 12: light, buzzer, messages and daily report.
• Weeks 13 to 15: field week, comparison, report and demo.

Path to a product

Construction firms, developers working for international clients, large farms and municipalities face growing pressure to protect workers from heat. A low-cost station, sold with a simple heat safety plan, fits that need. Buyers of export goods increasingly audit supplier working conditions, which adds demand. The project is a strong beginner showcase of sensing, calibration and safety-focused design.

Scope warning

This is a decision aid, not a medical device. Do not tell anyone they are safe; tell them the heat level and the recommended precautions. Do not attempt body temperature or heart rate sensing in the first version. Compare with a reference before claiming accuracy.

Learn what you need on Sefism

• Embedded systems and IoT roadmap: sensors, firmware and connectivity from scratch.
• Embedded systems and IoT cheat sheet: sensor interfaces and power basics.
• Arduino Documentation: a gentle start with sensors on the ESP32.
• MQTT Essentials: sending readings to a server.
• Should I start embedded systems with Arduino, ESP32, STM32 or Raspberry Pi?: picking your first board.

Get help with this idea

Calibration and clear alerts make or break this project. Tauseef can help you plan the comparison with a reference, find a site for the field week and keep the scope to one semester. If you are new here, create a free account first.

• Discuss this idea with a mentor
• Send a query to Tauseef
• Book a 1:1 session