UVA/UVB Rays & Sun Exposure & Change Ratio

Siesta (n.): “an afternoon nap or rest.”^[1] While perhaps not readily apparent, a siesta may be one of the easiest and most effective approaches to reducing UV damage to the skin.^[2] At midday, the intensity of UVB radiation tends to peak, increasing the harmful effects of sun exposure and thus, the need for protective measures.^[2-6]

Overview of UVA vs. UVB

First, a quick refresher on UVA and UVB. UVA has a longer wavelength at approximately 320-400 nm compared to UVB at 290-320 nm.^[3] With its longer wavelength, UVA penetrates both the epidermis and dermis. UVB acts more superficially, reaching only the epidermis. Commonly, UVB is considered the main cause of sunburn (B for burn), whereas, UVA is associated with aging (A for aging); however, the shorter wavelengths in the UVA spectrum can also contribute to sunburn. Exposures to both UVA and UVB radiation increase an individual’s risk of skin cancer.^[4]

Changes in the UVA/UVB ratio throughout the day

UVB radiation fluctuates due to multiple factors, one of them being the time of day. In contrast, the intensity of UVA radiation remains relatively consistent during daylight hours throughout the year.^[2-4] At sea level, the main factor in determining the UVA/UVB ratio is the angle of the sun above the horizon, called the solar elevation angle. Several studies have shown that the UVA/UVB ratio is highest in the morning and evening when the solar elevation angle is minimal, and lowest in the middle of the day when the solar elevation is maximal—typically between 10:00 am and 2:00 pm local solar time (fig. 1).^{[3, 5, 6]}  

Figure 1. Average UVA/UVB ratio throughout the day.^{[5, 6]} Note the high UVA/UVB ratios during the early morning and evening hours due to the lower UVB values during these times.^[5]

While the UVA intensity remains largely constant throughout the day, the UVB intensity tends to be lower in the early morning and late evening, peaking in the middle of the day (fig. 2).^[2-7]

Figure 2. Average UVB Radiation as a function of local solar time.^[7]

The shadow rule is a very practical tool you can use on sunny days as a reminder of UVB intensity. The shadow rule is as follows: UVB rays are least intense when an individual’s shadow is longer than their height and most intense during the hours when an individual’s shadow is shorter than their height. Thus, when a person’s height exceeds their shadow length, it is time to consider retreating indoors for a few hours!^[2]

Consider taking a siesta

So, what does this mean? While UV protection remains a priority at all hours, it is particularly critical during the midday hours due to the peak in UVB intensity, which is associated with a greater risk of sunburn. A siesta may initially seem like a foreign practice to some: who has time for midday naps? However, a looser definition of a siesta—taking a timeout from the sun, especially during the warmer months—can take many forms: abandoning direct sun to seek shade under a tree, taking a break from swimming to go inside for lunch, enjoying happy hour under an umbrella. Regardless of what it looks like, avoiding the sun for a few hours in the middle of the day is a practical and effective strategy to reduce UV-mediated skin damage.

Practical Tips

• UVA radiation is relatively constant during daylight hours while UVB radiation is greatest between 10:00 am and 2:00 pm local solar time. If possible, escape the sun for a few hours in the middle of the day to avoid the peak hours of damaging UVB rays.
• Despite lower UVB levels in the early morning and late evening, wearing a broad-spectrum sunscreen during these hours (and all daylight hours) is still necessary to protect the skin against UVA radiation.