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