Showing posts with label climate change. Show all posts
Showing posts with label climate change. Show all posts

Saturday, August 31, 2024

It’s Not The Heat – It’s The Wet Bulb Global Temperature (WBGT)!



Despite the current election cycle and massive denial of climate change by the MAGA party – the average temperature of the Earth is getting much higher. The ten warmest years on record are the last 10 years (see graph above).  That is corresponding with the expected melting of glacial ice, increasing sea surface temperatures, and higher sea levels. It is equally obvious that the necessary measures to reduce carbon dioxide in the atmosphere are not being taken.  We can therefore expect future generations to experience the burden of excessive heat, unstable weather patterns, flooding, crop shortages, and all of the economic and international instability that will occur.  I don’t think it is a stretch to see this as potentially catastrophic to civilization.

Of all those weather related phenomena – extreme heat and humidity are the number one cause of death.  Mortality due to heat stroke is increasing every year. This was the first year that I heard frequent stories about hikers perishing from the heat.  The CDC estimates that about 1200 people a year die from extreme heat and they have a web page dedicated to providing resources about this public health threat. The EPA has a site that looks at heat related deaths over the past 20 years and although they are higher – they discuss the issue of variable reporting, largely due to inconsistent criteria about what is called a heat related death.  There is a good chance that heat related deaths are underreported and attributed to other causes.

Modern reviews of heat stroke and heat related illnesses (1-3) suggest that there are two conditions that are clear emergencies and varieties of heat stroke – classic heat stroke and exertional heat stroke. Classic heat stroke develops in people exposed to heat who may be predisposed because of chronic illness, medications, and an environment that is excessively hot.  These patients are typically elderly.  Exertional heat stroke occurs when heat is generated by muscle exertion in hot weather and that leads to excessive body temperature. In both cases emergency cooling is a critical treatment along with fluid volume replacement if that is an issue.  Heat stroke is a multisystem inflammatory disease that leads to a combination of autonomic, cardiovascular, and metabolic responses that can lead to organ failure and death.  Mortality is high - 80% in classic heat stroke and 33% in exertional heat stroke (3) if there is no immediate treatment.  The issue of immediate treatment is problematic because heat stroke can cause compromised mental status including loss of consciousness – further exposing the person to prolonged heat exposure.  Preexisting cognitive compromise complicates both recognition of heat related illness and getting timely care.      

The issue of heat, humidity, and heat dissipation from the human body is an exercise in physical chemistry.  Evaporative cooling is one of the ways that mammals cool their bodies to maintain a stable body temperature.  In humans the other way is skin surface vasodilatation and heat transfer from blood.  Every time water changes phase (solid -> liquid -> gas) there is an energy requirement that is termed enthalpy in physical chemistry. The enthalpy or heat of transition from liquid to gas is 40.67 kJ/mol.  That means that we can calculate the amount of water necessary to maintain cooling at an amount of energy production. The relevant variables include how much energy the person is expending, ambient air temperature, and the relative humidity since the last two affect the amount of water that can transition to the gaseous phase.

All these variables can be considered form a meteorological viewpoint using the concepts of heat index and wet bulb global temperature. On this prototype site you can chose either value, click on your location on the map (it scrolls), and get the heat index or WBGT.  As an example, two days ago the temperature at my location went firmly into the danger zone as indicated by this tracing that I downloaded at the time. Suggested activity levels and precautions are available on the site for the WBGT values on the curve over the course of the day.  Another way to look at WBGT is that it is the temperature where evaporative cooling starts to fail and that is the measure of danger.





WBGT was recently assessed looking at wet-bulb temperature adaptability thresholds in health young research subjects (4).  Under experimental conditions they looked at subjects under moderate metabolic load and determined the critical wet-bulb temp beyond which the heat stress could no longer be compensated for.  The researchers looked at the proposed critical wet bulb temp of 35ºC (95 Fº)(threshold wet bulb temp) and discovered that it could be considerably lower and that it was unlikely that a single critical temperature for all conditions could be found.

Irrespective of the reporting issue, public health officials are concerned about ambient temperatures and the heat island effect of concrete in large cities where emitting energy can raise the ambient temperature up to 7 degrees hotter than the surrounding area.  They have identified high risk populations including the elderly, people with inadequate housing, and people with no air conditioning. That last category can rapidly expand whenever weather conditions effect power transmission and cut electrical power to large populations.  We just experienced those conditions in Minnesota shortly after a burst of peak temperature and humidity conditions.

Many people with chronic mental illnesses are in the high-risk category.  Compromised judgment whether due to effects on cognition or more focal problems with judgment and problem solving can lead to potentially fatal situations at either end of the temperature spectrum. During the recent heat emergency, I witnessed several people wearing excessive clothing and at the same time expending a lot of calories.  They were in a weather zone where the National Weather Service (NWS) suggested minimal activity. Chronic mental illness also creates the risk of no housing or inadequate housing. In extreme heat – the lack of air conditioning can create an emergency situation for many people.  

At the policy level (5) there have been some approaches to try to assure the safety of people that might experience exertional health stroke.  They are in the form of mandated breaks, access to water, and access to air conditioning.  They are far from universal and in some cases there has been political opposition based primarily on anti-science (climate change, human physiology) ideology and ignorance. It is a reckless approach to humanity but consistent with gun extremism views held by the same groups. 

At the individual level, psychiatrists need to be aware of their patients’ living conditions and their theoretical susceptibility to heat related illnesses.  That requires an integrated view of their current health status, medications, cognitive status, functional capacity, physical activity level, and specific access to air conditioning. Assertive Community Treatment (ACT) teams will have an advantage is knowing first hand what the living conditions are. In the past I have worked out a plan for people to go to an air-conditioned shopping mall depending on the weather conditions. Some people will find this difficult and could benefit from a visit and prompts from a case manager. A call list of people who may be at risk could be useful for case management teams.  As more municipal areas develop cooling shelters – a more formal referral process might be possible.

In the short term, a focus on the medical and social aspects of patient care will be necessary to mitigate the potential lethal risks of heat related illness. It is a necessary role for all physicians as the climate disaster unfolds.

 

George Dawson, MD, DFAPA

 

1:  Savioli G, Zanza C, Longhitano Y, Nardone A, Varesi A, Ceresa IF, Manetti AC, Volonnino G, Maiese A, La Russa R. Heat-Related Illness in Emergency and Critical Care: Recommendations for Recognition and Management with Medico-Legal Considerations. Biomedicines. 2022 Oct 12;10(10):2542. doi: 10.3390/biomedicines10102542. PMID: 36289804; PMCID: PMC9599879.

2:  Epstein Y, Yanovich R. Heatstroke. N Engl J Med. 2019 Jun 20;380(25):2449-2459. doi: 10.1056/NEJMra1810762. PMID: 31216400.

3:  Sorensen C, Hess J. Treatment and Prevention of Heat-Related Illness. N Engl J Med. 2022 Oct 13;387(15):1404-1413. doi: 10.1056/NEJMcp2210623. Epub 2022 Sep 28. PMID: 36170473.

4:  Vecellio DJ, Wolf ST, Cottle RM, Kenney WL. Evaluating the 35°C wet-bulb temperature adaptability threshold for young, healthy subjects (PSU HEAT Project). J Appl Physiol (1985). 2022 Feb 1;132(2):340-345. doi: 10.1152/japplphysiol.00738.2021. Epub 2021 Dec 16. PMID: 34913738; PMCID: PMC8799385.

5:  Burton A.  Energy justice for all: keeping disadvantaged populations cool in a heating world.  Federation of American Scientists: https://fas.org/publication/energy-justice-keeping-cool/

6:  Knochel JP. Heat stroke and related heat stress disorders. Dis Mon. 1989 May;35(5):301-77. PMID: 2653754.

This was the first comprehensive review I read on the topic.  I was a subscriber to Disease-a-Month and the Medical Clinics of North America early in my career - based on recommendations from an Internal Medicine resident who I worked with.  This remains an excellent review of the topic.  


Graphics Credit:

National weather Service (weather.gov) for all except for the North Carolina High School Athletics Association as noted.