Human health and ecological consequences of ozone depletion.

Take away ideas and understandings:

  1. Understand the effects of ozone absorption on UV-C, UV-B, and UV-A
  2. Understand the relationship between ozone depletion and UV-B radiation and where UV levels are presently relative to the past and projected future.
  3. Understand the projected future amplitudes and durations of enhanced UV radiation resulting from ozone depletion
  4. What are the projected UV percentage increases for the arctic, antarctic, and northern hemisphere average?
  5. Understand the human health hazards associated with elevated UV levels.
  6. Understand the mechanisms for eye and skin sensitivity to UV radiation.
  7. Understand the projected increases in eye and skin cancer for the next fifty years and how these projections were estimated.
  8. Understand potential impacts of elevated UV radiation on terrestrial and aquatic ecosystems

1.0 Overview

The health risks associated with ozone depletion will principally be those due to increased ultraviolet B radiation (UV-B) in environment, i.e., increased damage to the eyes, the immune system and the skin. Emerging knowledge of these risks led to the accelerated schedule of eliminating ODS (ozone depleting substances) included in the most current amendments to the Montreal Protocol. Quantitative risk estimates are available for some of the UV-B-associated effects, e.g., cataract and skin cancer. The following text and figures were largely drawn for sources in the CEDAC, Environmental Effects of Ozone Depletion, 1998 assessment. A comprehensive summary and review of UV-related health effects can be found in the World Health Org. (WHO) Environmental Health Criteria Monograph Ultraviolet Radiation, 1994.

2.0 Changes in Biologically active UV radiation reaching the earth's surface

All radiation consisting of wavelengths shorter than 280 nm is absorbed in the upper atmosphere; wavelengths longer than 320 nm are not significantly absorbed by ozone. Therefore, biological systems are vulnerable to wavelengths in the transitional region of 280 to 320 nm due to ozone losses.

The UV (ultraviolet) region of solar radiation covers the wavelength range 100-400 nm and is divided into three bands:

  • UV-A (315-400 nm)
  • UV-B (280-315 nm)
  • UV-C (100-280 nm)

As sunlight passes through the atmosphere, all UV-C and approximately 90% of UV-B radiation are absorbed by O2, ozone, as well as water vapor, oxygen and carbon dioxide. UV-A radiation is less affected by the atmosphere. Therefore, the UV radiation reaching the Earth’s surface is largely composed of UVA with a small UVB component. The region of concern for biological effects is the ultraviolet-B (UV-B) range from 280 to 320 nm.

The primary concern over ozone depletion is the potential impacts on human health and ecosystems due to increased UV exposure. Increases in skin cancer and cataracts in human populations are expected in a higher UV environment. Lower yields of certain cash crops may result due to increased UV-B stress. Higher UV-B levels in the upper ocean layer may inhibit phytoplankton activities, which can impact the entire marine ecosystem.

Stratospheric ozone levels are near their lowest point since measurements began, so current UV-B radiation levels are thought to be close to their maximum. Total stratospheric content of ozone-depleting substances is expected to reach a maximum before the year 2000. All other things being equal, the current ozone losses and related UV-B increases should be close to their maximum. Increases in surface erythemal UV radiation relative to the values in the 1970s are estimated to be:

- about 7% at Northern Hemisphere mid-latitudes in winter/spring;
- about 4% at Northern Hemisphere mid-latitudes in summer/fall;
- about 6% at Southern Hemisphere mid-latitudes on a year-round basis;
- about 130% in the Antarctic in spring; and
- about 22% in the Arctic in spring.

Even relatively small increases in UV-B radiation can lead to substantial biological effects.

Biologically active UV radiation: The overlap between the spectral irradiance F(l) and the erythemal action spectrum B(l) given by McKinlay and Diffey (1987) shows the spectrum of biologically active radiation, F(l) x B(l).

The area under the product function F(l) x B(l) is the biologically active dose rate. Thick lines are for a total ozone column of 348 DU, thin lines for 250 DU (one Dobson Unit, or DU, is defined as the height in milli-centimeters that pure gaseous ozone would occupy if compressed to 1013 hPa at 0°C, and thus equals 2.69x1016 molecules cm-2) (from Madronich and Flocke, 1997; text from CEDAC, 1998 assessment).

Note the increased UV-B radiation reaching the earth's surface under a diminished ozone column.

2.1 Estimating present and future changes in UV radiation due to ozone depletion

Relationship between ozone depletion and UV radiation

The evidence is overwhelming that under cloud-free skies UV-B radiation is controlled largely by ozone

The figure to the left shows the dependence of erythemal (skin-burning) ultraviolet (UV) radiation at the Earth's surface on atmospheric ozone, measured on cloud-free days at various locations, at fixed solar zenith angles.

The RAF, or radiation amplification factor, describes change in atmospheric transmission (transparency) to UV radiation for a given change in ozone column concentration.

UVbio ~ (Ozone)-RAF

An RAF of 1.1 means that the UV radiation goes up 1.1% for a 1.0% decrease in ozone concentration. Note the non-linear (power-law) nature of the relationship. What does this mean in terms of future decreases in ozone?

Future Ozone trends

Scenario for future changes in ozone and erythemally-weighted UV radiation at the Earth’s surface, at 45° N and 45° S.

UV radiation changes are estimated from ozone changes, which in turn are estimated from changes in atmospheric amounts of ozone-destroying substances (halocarbons). All other factors are assumed constant.

Future scenarios shown are based on current control measures (Montreal 1997 Amendments), with scenario A1 (baseline, solid curves) accounting for the fact that production of some ozone-depleting substances is currently already below the allowed maximum, while in scenario A3 (dashed curves) production is at the maximum allowed level. Dotted curves are the zero-emission limit (starting in the year 2000) and only illustrate the minimum delay time imposed by atmospheric processes (from Madronich et al., 1998).

Note that elevated UV levels (relative to 1970's baseline) will persist for another 50 years, and that the change is greatest for the southern hemisphere (8%).

3.0 Human Health risks of ozone depletion

UV exposure in humans is principally via the eyes and skin, with effects occurring as a result of the absorption of solar energy by molecules (termed chromophores) present in the tissues/cells present in these organs. The absorption of light energy leads to changes in these molecules that eventually can result in a biologic effect. The chain of events is:

Biol. UV absorption --> Biochemical change --> Ccellular death/alteration --> Organism response

Chromophores absorb light energy from the various wavelengths with differing efficiencies. This pattern of absorption is called an absorption spectrum and is characteristic of the type of molecule involved. The figure below shows absorption spectra for five of the chromophores present in skin and eye tissues that are thought to be important to the biologic effects of UV-B in humans and animals. These are DNA, tyrosine and tryptophan (two amino acids that are largely responsible for the UV absorbance of proteins), trans-urocanic acid (a molecule present in large amounts in the outermost layer of skin), and melanin (the principal pigment of the skin).

UV-B radiation and chromophore absorption. The gray area marks that part of the UV spectrum, wavelengths under 290 nm, which is not present in terrestrial energy. For all of the molecules except melanin, absorption efficiency drops rapidly within the terrestrial UV-B spectral region with little or no absorbance in the UVA spectral region (above 320nm).

Thus the increase in UV-B that accompanies ozone depletion will increase the amount of biologically active radiation present in ambient sunlight. Because of the biologic activity of UV-B, such increases are likely to have marked consequences for humans as well as other living creatures. Some of these consequences could be beneficial, e.g., a greater production of vitamin D in the skin of humans, but far more are likely to be detrimental.

Humans have three major organ systems whose cells and tissues are commonly exposed to sunlight: the eye, the immune system and the skin, and it is in these three systems that the effects of sunlight on health have been documented. The cells/tissues exposed in the eye are principally those associated with the cornea; the iris and the lens, those of the skin include the outermost layer of the skin, the stratum corneum, and the epidermis; and those of the immune system are the Langerhans (or antigen-presenting) cells that reside in, or migrate through the epidermis.

3.1 UV effects on Human Eyes

Enhanced UV radiation promote increased incidences of eye disease and blindness. In the US today, there is a clear north-south relationship in the incidence of eye diseases - residents in the sunny southwest are twice as likely to have cataracts that those living in the northwest.

The eyes are a principal route of exposure to UVR. As illustrated below, when sunlight (and the UVR it contains) impinges on the normal eye, the cornea is encountered first, then the lens, the vitreous humor, and the retina. Studies indicate that due to its absorption by various molecules in the cornea and the lens, most UVR never reaches the retina in the normal adult eye. In the case of ambient UVR (i.e., UV-B and UV-A), the shorter wavelengths are absorbed preferentially, with the cornea absorbing most of the radiation below 300 nm, and the lens absorbing almost all of the rest of the UVR below about 370 nm. Lens removal (as for the treatment of cataract) does place the retina at risk for UV damage; it is for this reason that many artificial replacement lenses are made with UV-absorbing materials.

Of all of the ocular diseases associated with solar exposure, those which affects the lens (cataracts) is by far the most important from a public health perspective. Characterized by a gradual loss in the transparency of the lens (due to the accumulation of oxidized lens proteins), the end-result is frequently blindness.

The economic and social importance of cataracts is enormous. It is be leading cause of blindness in the world, with public health care costs for cataract surgery in the U.S. exceeding $3 billion in 1992. With the prevalence of cataract after age 30 approximately doubling each decade, anything that accelerates onset by 10 years (e.g., the increase in UV achieved in moving from the northernmost to the southernmost regions of the US) is estimated to double the number of operations. The figure below shows a calculated projection of the number of excess cases of cataracts related to ozone depletion.

3.2 UV effects on the human immune system

Enhanced UV radiation reduced the body's ability to fight off infection. In humans, the skin is the principal barrier to the outside world, and thus the first line of defense against foreign agents that may threaten health. In order to fulfill this role, the skin hosts a number of cells from the immune system that can mount or modify immune responses against such 'foreign invaders' or against skin cells that have become 'strange', e.g., by virus infection or transformation into a cancer cell. However, to function optimally, the immune system needs to be able to discriminate between 'self' and ‘strange’ or 'non-self', and eliminate only the latter, especially if it is (potentially) harmful.

The skin contains a wide range of molecules, including both proteins and DNA, which undergo photochemical reactions upon absorbing UVR. The cell-surface proteins which are used to determine 'self' are evidently modified in such photochemical reactions so that, at certain UV doses, the skin becomes swamped with ‘non-self’ cells. Were the immune system to react to all of these cells, the resulting inflammatory response might compromise other important skin functions.

3.3 UV effects on skin diseases

Small amounts of UV are beneficial for people and essential in the production of vitamin D. UV radiation is also used to treat several diseases, including rickets, psoriasis, eczema and jaundice. This takes place under medical supervision and the benefits of treatment tend to outweigh the risks of UV radiation exposure.

The risk of UV-related health effects on the eye and immune system is independent of skin type. Overexposure to solar radiation may result in acute and chronic health effects on the skin, eye and immune system. Many believe that only fair-skinned people need to be concerned about overexposure to the sun. Darker skin has more protective melanin pigment, and the incidence of skin cancer is lower in dark-skinned people. Nevertheless skin cancers do occur with this group and unfortunately they are often detected at a later, more dangerous stage.

 

Right: Action spectra for dimer formation in epidermal DNA. An action spectrum is a measure of the relative effectiveness
of different wavelengths, within the spectral region of study, to produce a given response. Here, this particular molecule exhibits a peak response (sensitivity) at ~290nm, or within the UV-B range.

This plot demonstrates the sensitivity of cellular molecules to UV radiation.

 

Left: DNA dimer formation (a type of DNA damage) as a result of UV radiation exposure. Damage includes DNA strand breaks, cross-links, and dimer formation.

UV radiation can damage many cellular targets including the nucleic acids, proteins and lipids. For the non-solar UVC
wavelengths, DNA is clearly the most important target and many photochemical changes can occur as a result of direct absorption.

UV-induced photoproducts in DNA form part of the genesis of certain skin cancers such as basal cell and scamous cell carcinomas. The figure below shows a projection of the number of excess US cases of skin cancer resulting from enhanced UV radiation from a depleted ozone column. The projection considers several scenarios: no restriction (no action), the 1987 Montreal Protocol as proposed, and the subsequent London, Copenhagen, and Montreal amendments. What percentage is the percentage increase in from 2000 to 2050 using the most current amendment projection? How does this figure compare to the projected UV radiation increases figure at the top-middle of this page?

Want to protect yourself from increased UV radiation now and in the future? Here's a list of simple precautions.

4.0 Ecological effects of ozone depletion

4.1 UV-B effects on terrestrial plants (from the 1991 Report Summary, Teramura et al.)

The potential importance of current solar UV-B levels, even in the absence of further ozone reduction, has been demonstrated experimentally by reducing present day levels of solar UV-B radiation reaching the plants under investigation. It has been shown that plant growth, and in some cases photosynthesis, can be altered in seedlings. Whether this holds true for mature plants is not yet known, but these results indicate the potential importance of solar UV-B radiation even without ozone reduction.

Continued research on plant responses to UV-B radiation underscores the concern for agriculture, forestry, and natural ecosystems as the ozone layer is depleted. Yet, quantitative predictions are complicated by several factors, such as carbon dioxide concentration and temperature. These are relevant to global climatic change, and have been shown to influence the manner in which plants respond to increased UV-B radiation. For example, the stimulating effect of carbon dioxide enhancement may be altered by UV-B and may involve more than one photosynthetic process. Therefore, carbon dioxide may not fully compensate for negative UV-B effects. Temperature has also been shown to influence UV-B effects on growth and physiological processes in some of the species investigated. Other abiotic factors, such as heavy metals, may also modify UV-B influences on plants.

Recent measurements of the penetration of UV-B radiation into plant tissues have confirmed that internal changes in anatomical features and pigmentation vary among plant species. These changes result in alterations in plant response to UV-B radiation. Increased UV-B has also been shown to alter the biotic relationships of higher plants as demonstrated by the changes in plant disease susceptibility and the balance of competition between plant species.
More has been learned about the mechanisms of UV-B action. Both long-term UV-B irradiation of whole plants and short-term irradiation of chloroplasts may induce the synthesis of certain polypeptides in photosynthetic membranes that could play a role in mitigating UV-B damage. The influence of UV-B on growth appears to be mediated by phytohormones, either through photodestruction or enzymatic reactions. Whether other morphological responses to UV-B are also mediated by phytohormones remains to be demonstrated. Repair of DNA damage by photoreactivation has been clearly demonstrated in several plant systems. However, the limits of this photoreactivation capacity have not yet been determined.

Field and greenhouse studies have shown that growth and photosynthesis are negatively affected by enhanced UV-B radiation in some tree species such as the loblolly pine. Although little data are available for tropical species, preliminary greenhouse studies indicate that growth, photosynthesis, and yield decreased in some rice cultivars. Care should be taken in assessing and generalizing the results from particular plant species and cultivars to other species, since there appears to be a great range of UV-B responses among plants.

4.2 UV-B effects on aquatic ecosystems (from the 1991 Report Summary, D.-P. Däder et al.)

Aquatic ecosystems contribute more biomass (104 Gt/a) than all terrestrial ecosystems (100 Gt/a) combined. Recent work on UV-B effects has concentrated on inhibition mechanisms and field studies in the subpolar waters of Antarctica, because of its high productivity and the occurrence of the ozone hole over this region.
Phytoplankton organisms orient within the water column using external factors. However, mobility and orientation mechanisms are impaired by UV-B radiation. Because most organisms do not possess UV-B receptors, they cannot avoid deleterious wavelength radiation that (according to new measurements) penetrates deeper into the water column than what has been previously measured. New action spectra indicate that, in addition to DNA, other targets absorb UV-B radiation including intrinsic proteins of the photoreceptor and photosynthetic apparatus.
The inability to adjust their position within the water column causes massive inhibition of photosynthesis, measured both in field and laboratory studies. Only in a few cases have potential UV-B-inducible screening pigments been identified.

A large share of the nitrogen consumed by higher plants is made available by bacterial microorganisms, which have been found to be very sensitive to UV-B radiation. Losses in nitrogen fixation could be compensated by additional nitrogen fertilization. However, such actions could stress the capabilities of developing nations.
The role of DMS, released from plankton and macroalgae as aerosol and cloud nuclei, is of major concern. Most importantly, a UV-B-induced decrease in phytoplankton populations may have an impact on cloud patterns and concomitant global climate changes.

An increased understanding of Antarctic trophic dynamics suggests that the likelihood of direct UV-B radiation effects on consumers is small. Rather, it is the possibility of indirect effects that may significantly affect the Antarctic trophic structure, such as different species sensitivities to UV-B radiation, or decreases in total available primary production. Because more than 30% of the world's animal protein for human consumption comes from the sea, the human populations may also be affected by the direct and indirect consequences of increased solar UV-B radiation on aquatic food webs.

Another potential consequence of a decrease in marine primary productivity would be a reduction in the capacity of the ocean to absorb carbon dioxide. A hypothetical loss of 10% of the marine phytoplankton would reduce the oceanic annual uptake of carbon dioxide by about 5 Gt (an amount equal to the annual emissions of carbon dioxide from fossil fuel consumption). Uncertainties regarding the magnitude of increased levels of UV-B radiation on aquatic systems still remain, including problems of extrapolating laboratory findings to the open sea, and the nearly complete absence of data on long-term effects and ecosystem responses. Uncertainties and future research needs include adaptive strategies and the effects of cumulative UV-B radiative doses. Additional information is needed in several areas before a more reliable assessment of risks is possible.

Updated February 20, 2002

©2002 P. deMenocal (LDEO, Columbia Univ.)