Encyclopædia Britannica, Inc.
When we think of
pollution
, we usually think of
chemicals
introduced into places where they shouldn’t be:
fertilizer
and
pesticide
runoff,
diesel
fumes,
plastic
debris along roadsides, and
carbon dioxide
from
industry
and
transportation
. Pollution is often described as the addition of any substance or any form of
energy
to the
environment
at a rate faster than it can be dispersed, diluted, decomposed, recycled, or stored in some harmless form. So how does light pollution fit into this? Is it really pollution at all?
Light pollution
is unwanted or excessive artificial
light
. Our first impulse may be to call it a nuisance, since it may not seem to be in the same league as
air
and
water pollution
.
However, since light pollution disperses energy, and this energy disrupts the environment, it can legitimately be considered a form of pollution.
Light pollution affects the human environment in several ways. Perhaps most noticeable is that the night isn’t quite as dark as it used to be. The collective sky glow of
streetlights
, house lights, coach lights, and interior lighting brightens the night
sky
and makes
stars
harder to see. Paradoxically, artificial nighttime illumination can also make seeing things at ground level more difficult, as anyone who has ever been dazzled by the glare of an oncoming
automobile
’s headlights can attest.
There are other issues as well. Light interferes with
sleep
cycles, so people in areas polluted by urban sky glow may suffer interruptions of their normal
circadian rhythms
. The glare from a neighbor’s bright backyard floodlight can wake people from their sleep, and the constant background sky glow and misdirected light from nearby commercial parking lots and streetlights can also affect sleep patterns. The circadian rhythms of other
animals
?especially nocturnal ones like
migrating
birds
, foraging
mammals
, and
insects
?can likewise be affected. Many bird species fly long distances at night, some using the stars and the
Moon
to navigate, and sky glow can disrupt this process, causing many birds to become disoriented over urban and suburban areas. Consequently, some fly into illuminated towers and other brightly lit structures, such as houses and office buildings.
Fortunately, reducing light pollution may be easier than reducing other forms of pollution. In some areas, urban sky glow can be reduced simply by turning off lights when they are not needed or by eliminating them from certain places completely. In other areas, reducing light pollution may involve making physical changes to the light sources themselves, such as by adding shielding fixtures (called cutoff luminaires) to lighting systems to focus the light where it is needed.