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Glass As UV Light Filter
Does
Glass Block UV Light or Can You Get a Sunburn?
How Much UV Light Does Glass
Really Filter?
by Anne Marie Helmenstine, Ph.D.
You may have heard you can't
get a sunburn through glass, but that doesn't mean glass blocks all
ultraviolet, or UV, light.
The rays that lead to skin or
eye damage still can get through, even if you don't get burned.
Types of Ultraviolet Light
The terms ultraviolet light and UV refer to a relatively
large wavelength range between 400 nm and 100 nm.
It falls between violet
visible light and x-rays on the electromagnetic spectrum.
UV is described as UVA, UVB,
UVC, near ultraviolet, middle ultraviolet, and far ultraviolet, depending on
its wavelength.
UVC is completely absorbed by
Earth's atmosphere, so it doesn't pose a risk to your health.
UV light from the sun and
man-made sources are mainly in the UVA and UVB range.
How Much UV Is Filtered by Glass?
Glass that is transparent to
visible light absorbs nearly all UVB. This is the wavelength range that can
cause a sunburn, so it's true you can't get a sunburn through glass.
However, UVA is much closer
to the visible spectrum than UVB. About 75 percent of UVA passes through
ordinary glass.
UVA leads to skin damage and
genetic mutations that can lead to cancer.
Glass does not protect you
from skin damage from the sun. It affects indoor plants too.
Have you ever taken an indoor
plant outside and burned its leaves? This happens because the plant was
unaccustomed to the higher levels of UVA found outside, compared with inside a
sunny window.
Do Coatings and Tints Protect Against UVA?
Sometimes glass is treated to
protect against UVA. For example, most sunglasses made from glass are coated so
they block both UVA and UVB.
The laminated glass of
automobile windshields offers some (not total) protection against UVA.
Automotive glass used for side and rear windows ordinarily does not protect against
UVA exposure.
Similarly, the window glass
used in homes and offices does not filter much UVA.
Tinting glass reduces the
amount of both visible and UVA transmitted through it. Some UVA still gets
through, though.
On average, 60 percent to 70
percent of UVA still penetrates tinted glass.
Ultraviolet Light From Fluorescent Lighting
Fluorescent Lights do
emit UV light but usually not enough to cause a problem.
In a fluorescent bulb,
electricity excites a gas, which emits UV light.
The inside of the bulb is
coated with a fluorescent coating of phosphor that converts the
ultraviolet light into visible light.
Most of the UV produced by
the process is either absorbed by the coating or else doesn't make it through
the glass.
Some UV does get through, but
the UK Health Protection Agency has estimated that UV exposure from fluorescent
bulbs is responsible for only about 3 percent of a person's exposure to
ultraviolet light.
Your actual exposure depends
on how close you sit to the light, the type of product that is used, and how
long you are exposed.
You can reduce exposure by
increasing your distance from the fluorescent fixture or wearing sunscreen.
Halogen Lights and UV Exposure
Halogen lights release some
ultraviolet light and usually are constructed of quartz because ordinary glass
cannot withstand the heat produced when the gas reaches its incandescent
temperature.
Pure quartz does not filter
UV, so there is a risk of UV exposure from halogen bulbs.
Sometimes the lights are made
using special high-temperature glass (which at least filters UVB) or doped
quartz (to block UV).
Sometimes halogen bulbs are
encased inside glass.
UV exposure from a pure
quartz lamp can be reduced by using a diffuser (a lampshade) to spread out the
light or increasing your distance from the bulb.
Ultraviolet Light and Black Lights
Black lights present a
special situation. A black light is intended to transmit ultraviolet light rather
than block it. Most of this light is UVA.
Certain ultraviolet lamps
transmit even more of the UV part of the spectrum. You can minimize the risk of
damage from these lights by keeping your distance from the bulbs, limiting your
exposure time, and avoiding looking at the lights.
Most black lights sold for
Halloween and parties are pretty safe.
The Bottom Line
All glass is not created
equal, so the amount of ultraviolet light penetrating the material depends on
the type of glass. But ultimately glass offers no real protection against sun
damage to the skin or eyes.
Anne Marie Helmenstine,
Ph.D.
· Ph.D. in biomedical sciences
from the University of Tennessee at Knoxville - Oak Ridge National Laboratory.
· Science educator with
experience teaching chemistry, biology, astronomy, and physics at the high
school, college, and graduate levels.
· ThoughtCo and About Education
chemistry expert since 2001.
· Widely-published graphic
artist, responsible for printable periodic tables and other illustrations used
in science.
Experience
Anne Helmenstine, Ph.D. has covered
chemistry for ThoughtCo and About Education since 2001, and other sciences
since 2013. She taught chemistry, biology, astronomy, and physics at the high
school, college, and graduate levels. She has worked as a research
scientist and also abstracting and indexing diverse scientific literature for
the Department of Energy.
In addition to her work as a science writer, Dr.
Helmenstine currently serves as a scientific consultant, specializing in
problems requiring an interdisciplinary approach. Previously, she worked
as a research scientist and college professor.
Education
Dr. Helmenstine holds a Ph.D. in biomedical
sciences from the University of Tennessee at Knoxville and a B.A.
in physics and mathematics with a minor in chemistry from Hastings
College. In her doctoral work, Dr. Helmenstine developed ultra-sensitive
chemical detection and medical diagnostic tests.
Anne Marie Helmenstine, Ph.D.
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