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The Astrophysics of Sunburns

By Tim Hunter


1. Introduction – suntans and sunburns

Suntans project a look of health, vigor, and activity, though they may have been gained by sleeping at the beach or reading a book at a tanning parlor. A sunburn is painful to have and is even painful to behold in someone else. Suntans and sunburns represent different extremes of the skin’s reaction to the Sun’s rays. The physics and physiology of suntans and sunburns are fascinating and present us with an important message about the Earth’s environment.

2. The ultraviolet spectrum and the Sun

The ultraviolet (UV) spectrum extends from 10 to 400 nm1. It ranges from x-rays at its shorter end to visible violet light at its longer end. The ultraviolet spectrum is a continuum of wavelengths, but for purposes of discussion, it is further divided into the near UV (320-380 nm), the middle UV (200-320 nm), and the vacuum UV (10-200 nm) by physicists or UVA2 , UVB, and UVC (and sometimes UVD) by biologists (figure 1). UVA extends from 320 nm to 400 nm, and it is also known as the glass transmission UV. UVB extends from 280 nm to 320 nm, and it is known as the sunburn or erythema UV. UVC extends from 185 nm to 280 nm, and it is known as the bactericidal region.

The extreme ultraviolet region (EUV) is that portion of the electromagnetic spectrum from approximately 10 to 100 nm. EUV is an important portion of the spectrum for studying Solar, stellar, and galactic astrophysical phenomena, and it roughly represents that part of the ultraviolet spectrum which is ionizing. Photons having an energy greater than 13.6 eV (9.12 nm) can ionize hydrogen. For example, an absorber at redshift z having a neutral hydrogen column exceeding 2 x 1017 cm-2 is optically thick to photons with an energy greater than 13 eV. This produces a spectral discontinuity at the hydrogen Lyman limit at an observed wavelength of 9.12(1+z) nm (Madau, 2006). EUV, UVC, and UVD play little roles in the production of suntans or sunburns as they are completely absorbed by the atmosphere.

It should also be noted the definitions of UVA, UVB, UVC, and EUV vary slightly from sources to source. Since the electromagnetic spectrum is a continuous range of wavelengths, where one “region” or “band” of the spectrum ends and another begins is somewhat arbitrary.


electromagnetic spectrum

Figure 1a.  The electromagnetic spectrum. From: http://www.loc.gov/rr/scitech/mysteries/colors.html

figure 1b

Figure 1b. The various regions (bands) of the UV spectrum as defined by physicists (top) or biologists (bottom). From: Williams (2002)

Germicidal3lamps are designed to emit UVC radiation, which is an effective bactericidal3 agent (figure 2) (Williams, 2002; Zeman, 2005). As far as vision is concerned, the eye is totally insensitive to ultraviolet radiation. Unfortunately, UV radiation can damage the eye without one feeling any “heat” in the eye or seeing any unusual phenomena while being exposed to the radiation.

figure 2

Figure 2. The erythemal, germicidal, and visual effects of ultraviolet radiation. From Williams (2002).

UVA is only partially absorbed by the atmosphere and is the most commonly encountered form of natural ultraviolet radiation (figure 3a). It penetrates into the deeper layers of the skin (the dermis) and causes tanning and wrinkles (Ferrini, 1998). UVA light is commonly found in “black lights”, and most phototherapy and tanning booths use UVA lamps (Zeman, 2005).

The UVB region is of particular interest for this discussion, because it is best known for its harmful effects on the skin. It has enough energy to cause photochemical damage to cellular DNA, and it is not fully absorbed by the atmosphere. UVB is the main cause of sunburn, which is an acute4 reaction of the skin that following excessive exposure to ultraviolet radiation (UVR). UVB mainly acts upon the epidermis, the top layer of the skin.

The Sun radiates much as a blackbody with an effective temperature of 5770 K (figure 3b) (Rottman, 1997). The majority of the Solar emission is in the visible region. Only 10% of the total solar irradiance (TSI) is from the spectral band short of 400 nm. The band short of 300 nm accounts for only about 1% of TSI (Rottman, 1997). Radiation at 400 nm originates near the base of the solar photosphere, and emissions at 200 nm originate from near the top of the photosphere. Emissions between 100 to 160 nm originate from the Solar chromosphere, and those with wavelengths less than 100 nm originate from a transition region above the chromosphere or from the Solar corona (Rottman, 1997).



Figure 3a. Atmospheric absorption of ultraviolet radiation. From: http://www.loc.gov/rr/scitech/mysteries/colors.html

Solar Spectrum

Figure 3b. The Solar spectrum from 0.2 microns (200 nm) to 2 microns. From Rottman (1997).

One-half of the solar radiation is absorbed or scattered by the atmosphere before it reaches the ground (figure 4). Only 0.7% of the Sun’s total energy is in the form of UVB. While the Sun’s output of UVB is fairly constant, the amount reaching the ground is quite variable due to the changing atmospheric absorption as the Sun changes its altitude during the day (Schaffer, 1988). For visible light, the atmospheric extinction for one air mass, k, is typically 0.3 magnitudes. At UVB wavelengths, the upper atmosphere absorbs most of the radiation, producing a k of 4.6 magnitudes per air mass at 300 nm. Ultraviolet light is also scattered by air molecules absorbing another 1.2 magnitudes, and dust in the air scatters and absorbs 0.2 magnitudes. The total extinction of UVB is 6.0 magnitudes per air mass (Schaffer, 1988) (figure 5).

UV and Sun

Figure 4. From top to bottom, respectively, the relative brightness of the Sun, the zenith atmospheric transmission, the burning and tanning effect per photon, and the actual burning and tanning effect of ultraviolet radiation from the Sun versus wavelength in angstroms. From Schaffer (1988).


brightness versus altitude

Figure 5. The Sun’s brightness versus altitude above the horizon. From Schaffer (1988).

When the Sun decreases altitude in the afternoon from 600 to 300, its light and heat often do not noticeably change, but its UVB radiation reaching the ground decreases by a factor of 100, greatly reducing its sunburn potential. While the UVA is reduced less than the UVB, it is difficult to get a tan in the early morning or late afternoon. It is also difficult to get a tan in winter at temperate latitudes, because the Sun is simply not far enough above the horizon to enable significant amounts of ultraviolet radiation to reach the ground. According to Schaffer (1988), the winter noon Sun in Washington, DC, is only 280 above the horizon at midday, while the noon summer Sun is 740 above the horizon. It takes six hours of noon sunning in December to have the same effect of one minute in June.

Visitors to the tropics and visitors to high altitudes are at a heightened risk for sunburns. There is typically 25% less protective ozone in the atmosphere over the tropics, and the Sun is at a higher altitude most of the day. Grass, soil, and water reflect less than 10% of UV radiation, while beach sand reflects 15% and sea foam 25% (WHO, 2006). Snow is a near perfect reflector of UVB, easily burning an unsuspecting mountaineer. Also, there is somewhat less atmospheric protection at high altitudes because of less molecular UVR scattering. Sun burning takes place 40% faster at 10,000 feet (~3000 m) than at sea level. While an ordinary window pane cuts the UVB radiation by 90%, water droplets in clouds do not reduce it very much (Schaffer, 1988). This is why one can feel cool on a cloudy day and still receive a terrible sunburn.

Ultraviolet radiation (UVR) in space above the Earth’s atmosphere is intense, and astronauts must be protected from it. Astronauts would receive a severe sunburn in 10 seconds if not protected. This is 250 times faster than on a Florida beach at noon in June (Schaffer, 1988). The biological effects of ultraviolet radiation can be exacerbated by a number of common medications, including birth control pills, tetracycline and other antibiotics, antidepressants, and some cosmetics. Protection against UVR is provided by clothing, glass, acrylics, plastics, and sun-blocking lotions as discussed below (Zeman, 2005).

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[1]Ultraviolet light was discovered by Johann Wilhelm Ritter (1776-1810) in 1801 after hearing of William Herschel’s discovery of infrared light beyond the red end of the visible spectrum.  Ritter noted blue light caused silver chloride to turn black more efficiently than red light.  He also discovered that “light” beyond the violet portion of the visible spectrum displayed an intense reaction in this regard.  From: http://coolcosmos.ipac.caltech.edu//cosmic_classroom/classroom_activities/ritter_bio.html.

[2] UVA, UVB, and UVC are sometimes used in a hyphenated form, such as UV-B.

[3] “Germicidal” or “bactericidal” agents are chemicals, medications, or forms of radiation that kill bacteria.  These agents often kill viruses as well.  Bacteriostatic agents are those which stop the growth of bacteria but do not necessarily kill them. 

[4] “Acute” is a medical term for a rapid (minutes to hours) effect while “chronic” is a medical term for a long duration (weeks to months) effect. 



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