Stay hydrated, get some shade, and take breaks.
The original dosimeter was made to detect radiation exposure. The original types, disposable, worked by making it apparent how much chemical change had resulted from ionization. Modern radiation dosimetry uses electronic sensors to achieve the same purpose, but are more accurate and reusable. (See OSHA: “Ionizing Radiation”: Dosimeters.)
There are also dosimeters for noise. They detect how much loud noise a worker is subjected to over time, in order to help protect their hearing. (See OSHA: “Occupational Noise Exposure”.)
And there is dermal dosimetry, which isn’t broadly developed, but is meant to measure how much of some environmental contaminant or toxin gets deposited on a worker’s skin. (See OSHA: “Dermal Exposure”.)
But I haven’t found much about heat dosimetry. (See OSHA: “Heat Illness Prevention” for general information about occupational heat stress.)
The labor problem of extreme heat may require the creation of a new type of dosimeter. This might be called thermal dosimetry, but that already refers to quantifying how much heat a medical patient receives in therapeutic hyperthermia—the use of heat to treat cancers.
Heat dosimetry would need to monitor a person’s time in environments that harm their body’s ability to regulate temperature. It’s a complex problem, which has been avoided in favor of self-monitoring and following general guidelines (access to shade, water, and breaks; monitoring environmental conditions generally, including sunlight and the heat indexed temperature).
It is somewhat complicated by the concept of acclimatization. People who are exposed to hot environments see their bodies adapt to be better suited to work in those environments. A gradual increase in exposure over a week or two makes the body better at sweating and maintaining body temperature in hot environments. (See CDC: NIOSH: “Heat Stress Acclimatization”.)
Would a theoretical heat dosimeter need to be calibrated or toggled for acclimatization? Only to a point. There are limits to how well the body can adapt to extreme heat and humidity, and so at levels beyond those, measuring strict exposure time would be enough.
Heat is a combination of several factors. Air temperature is the most obvious, but sun exposure and humidity are very important as well. There is also the surroundings, what the person wears, general fitness, and acclimatization. Finally, the type of activity the person engages in has a major effect in how rapidly they heat up.
If a heat dosimeter existed, it would need at least three components. One would monitor sunlight, another ambient temperature, and the third would measure humidity. Some reference values would be needed, but calculating the heat index with an adjustment for sun exposure would probably add up to a basic dose-check that could be used.
Unlike other exposure dosimeters, the big difference with heat is rapid recovery time—if the person has not been overexposed. While radiation, noise, and toxins provoke absolutely cumulative harms, heat exposure (up to a limit) can be tolerated with rest and removal from bad conditions. That’s an argument both in favor and against using dosimetry: it’s a recoverable condition, and regular breaks with relief from the heat are sufficient protection, but if only vitals are monitored, waiting for warning signs may be harmful.
On the other hand, measuring symptoms might work better than the traditional dosimetry approach. Monitoring the heat stress symptoms of the person may prove a better fit than trying to treat the problem like typical exposure dosimetry. That would include heart rate, body temperature, and hydration level.
Measuring acclimatization is important as well, and could probably be derived from the other measures. A person’s level of acclimatization can vary from day to day, depending on other stresses they may be under (including how much sleep they’ve had, dietary fluctuations), so acclimatization shouldn’t be treated as a binary.
Better heat-monitoring tools are definitely needed. Whether it’s practical or desirable to monitor exposure to environmental heat remains to be seen. Monitoring vitals directly, rather than exposure, has the benefit of alerting to other conditions and issues. Having regular breaks and access to adequate shade and cold water are enough to prevent most heat issues up to a point. Beyond that point, measuring heat doses may be important.
There are other alternatives to working in hot environments. Night work or temporarily and partially conditioned environments may be possible, but come with their own challenges. Night shifts cause sleep-related stresses and has some extra costs associated with it. Partial conditioning of a work environment requires extra labor and costs. How those costs stack up against working with heat isn’t clear.
For example, installing fixed poles for stretching tarps or canopies over a field to provide shade during fieldwork might be a practical mitigator for farms. Roadwork can benefit from night shifts, as it reduces traffic woes as well.
All that being said, it’s likely some companies have or will explore heat dosimetry. And it’s likely that at least some workers would benefit from it, even in the presence of direct monitoring of their vital signs and health. It will be interesting to see what the devices look like and how they operate.
It’s sure that the world is getting hotter from climate change, and we have to keep looking for ways to adapt.