On Heat and Humidity

With temperatures reaching 110F (43.3C) for over three consecutive weeks in Arizona, a heat dome gripping southern Europe, and the hottest day / month / year ever recorded in the world in July, reality these days looks eerily like the incipit of the novel “The Ministry for the Future” by Kim Stanley Robinson. While we have written about other aspects of climate change in these notes before, we have not yet addressed the core aspect of climate change: the heat itself. What are the consequences of too much heat on humans, animals, and societies? In other words, how hot is too hot?

It’s been known for some time that heat alone is not a good predictor of how the body will react; physiologic response is also a function of humidity. The combination of the two, measured as “wet-bulb temperature”, allows a better measure of tolerance for the human body. The wet-bulb temperature (T_w_) is the temperature read by a thermometer covered in water-soaked (water at ambient temperature) cloth (a wet-bulb thermometer) over which air is passed. At 100% relative humidity, the wet-bulb temperature equals the air temperature (dry-bulb temperature); at lower humidity, the wet-bulb temperature is lower than the dry-bulb temperature because of evaporative cooling. Intuitively, if a thermometer is wrapped in a damp cloth, it will behave differently than if it isn’t. The drier the air is, the faster the water will evaporate. The faster water evaporates, the lower the thermometer's temperature relative to air temperature. This explains the difference in apparent temperature for humans. The drier the air, the easier it is for extra water to evaporate. The result is that sweat evaporates more quickly in drier air, cooling down the skin faster. If the relative humidity is 100%, no water can evaporate, and cooling by sweating or evaporation is not possible.

A much-cited study published in PNAS in 2010 asserted that a wet-bulb temperature of 95F/35C–95F/35C at 100% humidity, or 115F/46.1C at 50% humidity–would be the upper limit of safety, beyond which the human body can no longer cool itself by evaporating sweat from the surface of the body to maintain a stable body core temperature. This paper describes the effect of a warming world and the portion of the earth that would no longer be suitable for human living. However, a more recent study further tests the actual effect of heat and humidity on human health. To answer the question of “How hot is too hot?” the Noll Lab for Human Performance Research at Penn State University set up a control environmental chamber to test young, healthy adults. Participants swallowed a small thermometer pill that continuously monitored their core temperature while subjected to minimal activity that would mimic daily living. A range of temperatures and humidity was then applied to the room and the rise in the core temperature of participants was monitored. Scientists running the study coined the zone at which the person’s core temperature continuously rises as the “critical environmental limit.” Below this limit, the body can maintain a relatively stable core temperature over long periods. Above the limit, core temperature rises continuously and increases the risk of heat-related illnesses with prolonged exposure. The study reveals starting at a T_w_ of 87F/31C and humidity above 50%, the body often cannot successfully thermoregulate and the core temperature creeps up dangerously.

In 2021, the Lancet published a helpful review on extreme hot weather and the associated health risks. In short, the human body responds to heat stress in two primary ways:
– Blood flow is redistributed towards the skin through vasodilation to improve heat transfer from heat-generating muscles to the skin, in contact with cooler air, and subsequently to the environment. This redistribution and vasodilatation require the heart to increase activity and can cause stress.
– Sweat is secreted onto the skin, evaporating and removing body heat. The reduction in body fluid through perspiration further increases cardiovascular strain and can lead to acute kidney injury and failure.

The brain regulates these physiological heat loss responses, with additional input from temperature-sensitive nerve cells in the skin and throughout the body. The inability to thermoregulate and cool down the body’s core temperature can result in various illnesses, including heat cramps, heat exhaustion, heatstroke, and hyperthermia from extreme heat events. Extreme heat can also worsen chronic conditions such as cardiovascular disease, respiratory disease, cerebrovascular disease, and diabetes-related conditions. Some populations are less able to thermoregulate—children, pregnant women, and older adults—making them more susceptible to adverse effects.

Heat-related illnesses and deaths are becoming more frequent (although collecting these data could be much improved) with the rise of global temperatures. While technologies and recommendations to counter the effect of the heat exist at the individual level—avoid heat exposure, hydration, frequent breaks from activity—efforts to adapt to the new climate reality need to be implemented at societal and global scales. Public health measures will have to be implemented by cities by creating ways to reduce heat concentration and by companies to adapt labor practices for agricultural and construction workers, to name a couple. The ever increasing weight of the scientific record along with Robinson’s visceral description of extreme heat and its consequences should compel us to take action today.

– Jonathan Friedlander, PhD & Geoffrey W. Smith



First Five
First Five is our curated list of articles, studies, and publications for the month. For our full list of interesting media in health, science, and technology, updated regularly, follow us on Twitter or Instagram.

1/ When the immune system warns against dangerous foods
Despite their role in promoting allergic symptoms in allergic patients, mast cells are a poorly understood part of the immune system. A recent report published in Nature uncovers a surprising new function of mast cells: they act like a sensor that helps to avoid contact with allergens. Mice lacking mast cells did not avoid foods they were allergic to over innocuous foods, while wild-type mice avoided foods that would cause them harm.

2/ The good mutations
While most mutations are deleterious and cause some malignancies, some are surprisingly helpful. Scientists identified a new type of mutated hematopoietic stem cell that was inversely associated with Alzheimer’s Disease (AD). After further analysis of a larger population, they concluded that the mutations were protective against AD dementia.

3/ Personalized medicine for microbes
The relationship between the host and its microbiome is an ever-evolving battle, the immune system pushing against microbes and microbes pushing against the immune system. A recent study in Science describes how antimicrobial peptides (AMPs), once considered to have broad activities, can be targeted to specific microbes present in the environment to prevent infections. The gene encoding these AMPs show rapid evolution and demonstrates surprising ecological adaptation of the immune system.

4/ Cells are rational economic agents
Researchers have combined economic theory with biology to understand how natural systems respond to change. They reported in a recent publication a similarity between consumers' shopping behavior and the behavior of metabolic systems. The team predicted how different metabolic systems responded to environmental changes by using methodologies used in economics. Using tools from other academic fields, they managed to describe previously unknown universal properties of metabolic systems.

5/ A supplement that shows its muscles
If you don’t take advantage of your “good mutations” described above, there is another way. A recent study published in Cell revealed that beta-hydroxy beta-methylbutyrate, also called HMB, a staple of a bodybuilder's diet, may help protect memory, reduce plaques, and ultimately help prevent the progression of Alzheimer's Disease. After your workout, don’t forget to hydrate, drink your protein shake, and take your HMB!



Engineering Biology
Jacob Oppenheim, PhD, an Entrepreneur-in-Residence at Digitalis Ventures, writes a blog entitled Engineering Biology available on the Digitalis Ventures website.

This month, Jacob discussed i) the challenges in building meaningful Machine Learning (ML) models in medicine, detailing algorithms poorly fit to the contexts and routines of healthcare data, and ii) how ML is not an oracle but a process efficiency in biology, useful only when embedded into tightly defined scientific and operational workflows.



Digitalis Commons
Catalyzing Health.‍

Digitalis Commons is a non-profit organization that builds frontier-advancing, scalable solutions that have an outsized impact on important problems in health and health care.

We are thrilled to announce that Digitalis Commons has entered a Partnership Intermediary Agreement to provide commercialization services to the Advanced Research Projects Agency for Health’s (ARPA-H) Project Accelerator Transition Office (PATIO). This first-of-its-kind agreement aims to speed up and smooth the process of getting breakthrough innovations in health to the American public.

Digitalis Commons is hiring to support our work with ARPA-H. Please visit the Commons website to learn more.

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