Wet bulb temperatures

By: Michael L. Tan - @inquirerdotnet - Columnist/CDN Digital | May 16,2023 - 07:00 AM

Last May 2, I wrote a column about the heat index, which measures both air temperature and relative humidity for a more accurate assessment of the possible risks coming from heat.

Over the weekend, I learned about another measure, called the wet bulb globe temperature index, often shortened to wet bulb temperature. A simple version involves the use of a thermometer with a wet bulb to take the temperature. This method simulates the way the human body tackles heat, which is sweating to cool the body.

Meteorologists, our “weather people,” have been taking these measurements for years but it’s only in recent years that governments are waking up to the need to intervene, with public alerts about the day’s high heat index and, now, a wet bulb temperature.

More complex calculations of the wet bulb temperature go beyond heat and temperature, adding factors like sun angle, cloud cover, and wind speed. (You know what it feels like, turning on an electric fan on a very hot day, and not getting too much comfort.)

Yesterday, Hong Kong government guidelines related to the wet bulb temperature went into effect, giving three possible “alerts” with guidelines on what companies will have to do for outdoor workers as well as those working indoors without air conditioning.

The guidelines categorize workers according to their physical workload. Examples of “light” workers are security guards. “Moderate” workers include cleaners and delivery persons and “heavy” workers include porters and those working in construction.

Under an “amber” alert, which is the lowest wet bulb temperature classification, light workers can continue with their regular routines. Moderate workers have to take a 15-minute break for every 45 minutes of work and heavy workers must alternate 30 minutes of work and 30 minutes of rest.

When there’s a “black” alert, the highest danger classification, light workers must alternate 30 minutes of work with 30 minutes of rest. Moderate workers have to have 45 minutes rest for every 15 minutes of work and for heavy workers, work is totally suspended.

Reactions were predictable. Labor unions welcomed the guidelines while employers said this would adversely affect productivity.

The requirements are not legally binding but if a worker suffers heat stroke and takes the employer to court, the guidelines on wet bulb temperatures can be invoked by judges for rulings on the companies’ liabilities.

Expect more and more governments to issue similar guidelines. As early as 2012, China already issued very simple ones: all outdoor work stops if the temperature is more than 40 degrees Celsius. If the temperature is between 37 and 40 degrees Celsius, outdoor work cannot be more than six hours a day.

Even without laws and guidelines, people will find ways. I’m always hearing complaints about how slow construction work is in the Philippines but we shouldn’t be surprised—construction workers will slow down on their work on their own on hot days. Wouldn’t it be better if they are allowed to take more breaks, depending on the heat index or wet bulb temperature?

My interest in heat stroke comes from being an educator—I’m hearing of more and more cases of student-athletes suffering heat strokes in recent months, including one death of a high school student.

We still need more research to understand how humans, animals, and plants handle heat. Remember, it wasn’t until 2021 that a Nobel Prize was awarded for the fairly recent discovery of body receptors for temperature and touch. More than “thermoception,” we need to better understand thermoregulation, how we function (or don’t function) with extremes of temperature.

I was looking up wet bulb temperature monitoring in the Philippines and could find only one site (meteologix.com) with daily reports from different sites. Several places in the Philippines have measurements that go beyond 30 degrees Celsius for wet bulb temperatures, which comes close to 31.6 degrees Celsius, the point at which the body can no longer regulate the heat.

Schools and work sites should initiate education on managing heat, from the need for constant hydration to using protective clothing. I sweat seeing people wearing black jerseys on a hot day! The education should also include being able to tell when someone is succumbing to the heat; I suspect many of us can’t even tell when the heat is injuring … or killing someone.

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