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Draft Briefing Paper 03-1-95
IMPLICATIONS FOR HUMAN HEALTH
Stratospheric Ozone Depletion and Enhanced
Exposure to Ultraviolet Irradiation
Summary of the Problem
Stratospheric ozone depletion and increased incident ultraviolet irradiation
at surface is most likely to result in an increased incidence of skin
cancers including melanoma, increased frequency of cataracts, accelerated
skin aging, and possibly alterations in immunological response.
Canadian Recommendations 1-3
- Complete elimination in the production and consumption of carbon
tetrachloride and methyl chloroform by 1995, chlorofluorocarbons (CFCs)
by 1997, and halons by 2000;
- Control on the use of hydrochlorofluorocarbons (HCFCs), and reduction
in the production and consumption of HCFCs by 2010;
- Provision of sufficient support for initiatives in the reduction,
recovery, recycling, replacement and safe destruction of CFCs and halons
from commercial, household and mobile refrigeration systems;
- Provision of sufficient support for research on ozone depletion and
the human health effects of increased UV-B exposure;
- Establishment of Canadian ozone and UV-radiation level monitoring
networks, Canadian UV-B standards for sun glasses, suntan and sunscreen
lotions, and public education programs for safe exposure to the sun;
- Development and implementation of the public education programs for
safe exposure to the sun, and responsibilities in eliminating ozone-depleting
substances.
Ozone Depletion
Stratospheric ozone depletion is not to be confused with tropospheric
ozone accumulation; ozone is an air pollutant in the lower troposphere
and a greenhouse gas throughout the troposphere, but a vital protective
shield against potentially lethal UV-B irradiation in the stratosphere.
Stratospheric ozone is regenerated by photolysis of oxygen and is minimally
affected by migration of tropospheric ozone upwards into the stratosphere.
The stratospheric ozone layer was first observed to be thinning over
Antarctica in 1985.4 Repeated observations
have confirmed the attenuation and charted its progress.5
In 1956-76, the first 20 years of observations from space, the ozone layer
was stable; since then, it has declined in thickness over Antarctica from
about 300 to between 125 and 200 Dobson units (units of concentration
in a vertical atmospheric column under standard conditions). Substantial
attenuation took place in the southern hemisphere close to Antarctica,
that is, the "ozone hole" places southern regions of Australia, New Zealand,
Chile, and Argentina; there is also some attenuation in the northern hemisphere.6-8
On average, the global stratospheric ozone layer has declined about 1.5%
per decade during 1958 to 1991. Attenuation varied by season and latitude,
being greatest in winter and polar regions. As compared to a baseline
at the same season over the period of 1957 to 1980, about 11% to 17% of
ozone depletion in early 1993 has been observed in Canada, and about two-third
of this loss has occurred since 1991.9
Ozone depletion could enhance ultraviolet (UV) flux, particularly the
shorter wavelength UV, reaching the Earth's surface. Significant increases
in UVB irradiation at surface have been found at all latitudes except
the tropics over 1979 to 1992 by means of a radiative transfer analysis
of satellite-based ozone measurement.10 In
the Northern Hemisphere the process is not as far advanced, at present.
The increasing tropospheric pollution in urban areas of industrial nations
may mask the effects of stratospheric ozone depletion.11-14
However, the polar and subsequently more generally northern regions of
Canada, Russia, and Scandinavia would be affected first in the Northern
Hemisphere, along with alpine regions.15-17
Various industrial chemicals may contribute to ozone depletion, including
CFCs, halons, carbon tetrachloride, methyl chloroform, hydrochlorofluorocarbons
and methyl bromide.18 CFCs alone account for
about 80% of total ozone depletion.
Chlorofluorocarbons release chlorine by photolysis in the atmosphere;
this free chlorine scavenges ozone and destroys it.19
One chlorofluorocarbon molecule may destroy as many as 10,000 ozone molecules.19-20
Recent studies have shown a direct and close correlation between cloud
layers where CFC-associated chlorine is generated and short-term ozone
depletion in localized areas over the Arctic; these are interpreted as
having caught "in the act" the initial stages of ozone depletion as they
evolved.21 Release of chlorofluorocarbons into
the atmosphere occurs through industrial activity, leaks, or the decommissioning
of old refrigeration and air conditioning units, as well as by use of
aerosol cans using the compounds as propellants.22-26
The loss of ozone may be, in part, caused by a huge mass of volcanic aerosols
(sulfate aerosols) of which are injected into the lower stratosphere.9,27-28
Substantial progress on curbing chlorofluorocarbon generation and release
on a national level has already been made, first with the Vienna Convention
on the Ozone Layer and later with the much more stringent Montreal Protocol.29
However, given the long half lives of the chlorofluorocarbons (75 years
of more), the emissions already released are expected to persist in their
ozone-depleting activity at significant levels well into the 22nd century.20
As well, carbon dioxide accumulation tends to offset ozone depletion,
introducing another variable that is poorly understood.30
Ultraviolet Irradiation
The impacts of stratospheric ozone depletion on human health can be direct
and indirect effects. An increase of UV radiation due to ozone loss may
result in the increase of tropospheric ozone of which would rise a prevalence
of respiratory diseases. An increase of tropospheric ozone as well as
UV radiation may impair crop yields and affect the biota in food chain
and other ecosystems.31-33
The consequences of stratospheric ozone depletion are thought to be directly
the result of increased exposure to ultraviolet light.34
Attenuation may occur from dust in the atmosphere, so that considerable
local variability may occur. Ozone in the stratosphere, particularly,
absorbs ultraviolet light completely in the UV-C range (200-290 nm) and
a large proportion in the UV-B range (290-320 nm). Intercellularly, UV-A
(320-400 nm) is absorbed by proteins and DNA, UV-B by all nucleic and
by aromatic acids, and UV-C by all cellular constituents; absorption may
lead to breakage of covalent bonds in critical macromolecules and to carcinogenesis,
accelerated aging, and cataracts.10,20,35-66
Of great concern is accumulating evidence for suppression of cellular
immunity by UV indicator.20,61-72
Non-melanoma skin cancer (particularly squamous cell carcinoma and actinic
keratitis, a premalignant condition) has long been considered as one of
the human health effects of increased ultraviolet irradiation due to ozone
depletion. The epidemiological evidence of non-melanoma skin cancer indicates
mainly indirect and weaker aetiological role for sun exposure in individuals.45
Those at greatest risk for direct effects of UV exposure on skin are people
with fair skin who sunburn easily, including among them many of Celtic
origin and persons with freckles and red hair. There are many rare skin
conditions that also predispose to UV-induced injury, including albinism
and ataxia-telangiectasia.
There is projected to be an increase in incidence of approximately 2%
for basal cell carcinomas and 1.5% for squamous cell carcinomas for every
1% reduction in ozone concentration.63 However,
the so-called amplification factor decreases with increasing latitude,
markedly above 30°C. This implies that residents of the far North
may, paradoxically, be at less increased risk for the carcinogenic effect
of increase ultraviolet penetration than those at lower latitudes, where
most of Canada's population lives, despite the publicity surrounding thinning
of the ozone layer over the poles.64-65 A reduction
in the ozone shield effect of 50% would increase the exposure of most
Canadians to levels similar to or exceeding that of Mediterranean countries.66
The effect in tropical latitudes would be proportionately greater.
Malignant melanoma is less common but more fatal than non-melanoma skin
cancer. Human data show a clear association between melanoma and sun exposure.40.42
Whether sunlight acts as an initiator or a promoter remains obscure. At
present, studies suggest that the biggest risk may be years after skin
damage during incidents of sunburn in early life.
An increase risk of the formation of cataract and retinal degeneration
is possible link to cumulative sun exposure. Presumably, oxygen free radicals
generated by UVB might damage the lens protein.
Helper T-cell function appears to be suppressed by UV indication, leading
to a potentially large number of possible health effects from increased
incidence and severity of systemic malignancies to the possibility of
compromised host defences against some types of infection.63
Cosmetically significant effects may include accelerated aging of skin
and perhaps increased frequency of pterygia.61
Usage patterns of protective clothing sunscreens, and eyeglasses, both
tinted and clear, may change a great deal with recognition of the problem
but measures taken by individuals to protect themselves are likely to
be incompletely effective even so. Commercial sunscreens may be effective
against UV-induced sunburn given a high enough rating and suitable exposure
periods,74 but their effectiveness against
UV-induced cancer is unproven.75 Other measures,
such as dark or reflective clothing, parasols, protective sunglasses,62
and hats are unlikely to be readily accepted as permanent fashion changes
in a world where clothing fashions change rapidly.
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