It’s not just the heat. It’s the sun.

As Arizonans, we’re experts on “hot.”  We know, for example, that it’s hotter in the sun than in the shade.  We know it because we can feel it.  A few years ago, researchers at Arizona State University surveyed thousands of people over the course of a year on how they felt temperature-wise at that moment.  The choices were “very cold,” “cold,” “cool,” “slightly cool,” “neutral,” “slightly warm,” “warm,” “hot,” and “very hot.”  The summertime responses differed by nearly a whole level based on whether the person was in the sun or the shade at that moment: the responses averaged “hot” in the sun and only “warm” in the shade.

Being in the sun doesn’t just affect how we feel–it affects how well our bodies perform.  In a recent study, researchers found that solar radiation dramatically degraded physical capacity at high temperatures above and beyond the effect of heat alone.  The researchers monitored the heart rates of a group of young, well-hydrated male subjects in hour-long sessions on treadmills.  Trials were conducted at various temperatures and humidity levels with the subject either wearing no shirt or with a t-shirt and a coverall.  In some trials, participants walked under solar lamps producing sun-like radiation at levels typical of the hottest part of a sunny day.  In others, the lamps were turned off.

As expected, the subjects’ physical capacity was highest during the trials in lower temperatures, regardless of other conditions.  At 77 degrees Fahrenheit in dry climate conditions (20% humidity), participants retained on average about 90% of their full physical capacity–a little more for those who were shirtless, a little less for those wearing a coverall.  As temperatures increased, physical capacity dropped substantially, regardless of conditions: for example, at 95 degrees and low humidity, capacity dropped to around 80% on average. 

Solar radiation did not have much effect on the capacity of the shirtless subjects in these conditions.  Above 95 degrees, however, the effects of solar radiation in dry conditions increased exponentially and the bare-chested participants began to wither under the lamps.   In a dry heat of 113 degrees, shirtless participants without solar radiation retained over 60% of their capacity, but those exposed to solar radiation retained only around 25% capacity.  (In humid conditions, you can forget about doing anything in such high temperatures; humidity substantially degraded physical capacity at all temperatures, and work capacity was so highly degraded at 95 degrees that the researchers didn’t even conduct any tests above that.)   

Clothing coverage provided partial protection from the solar radiation: in the dry conditions, those wearing a coverall retained about 50% capacity at 113 degrees. Still, even when wearing the protective coverall, participants had less capacity at 104 degrees with solar radiation than they did at 113 degrees without solar radiation.  Similarly, fully-clothed participants in the dry conditions had more physical capacity at 104 degrees without solar radiation than they had at 95 degrees with radiation.  

Bottom line, this evidence suggests that on a hot summer day in the desert, shade is roughly equivalent to around ten degrees in air temperature in terms of the physical activity that can be tolerated in the shade vs. the sun.  Indeed, the National Weather Service advises that full sun can increase the Heat Index–one measure of how hot it really feels–by up to 15 degrees.  While stripping down might help a little if it’s not too hot, it becomes counterproductive on very hot days and at any rate is risky and not recommended because it increases the risk of skin cancer.

Shade won’t make the summertime heat go away.  But Arizona policymakers should not underestimate the power of shade to make working, walking, cycling, and other necessary and desirable daytime outdoor activities realistically possible during the dangerously hot summer months.

Slade Smith

Slade is Assistant Director of the Applied Health Policy Institute and an Arizona attorney. He is a 2017 graduate magna cum laude of the University of Arizona James E. Rogers College of Law and co-recipient of the 2017 Ralph W. Aigler Memorial Award for his professional and scholarly contributions to the college. He is from Columbus, Ohio, and now lives in Tucson, where he regularly enjoys outdoor activities in southern Arizona's beautiful and varied desert and mountain landscapes.