The adverse impact of daylight on health are due to prolong exposure to UVR (wavelengths between 200 - 400 nm, see Figure 2.1). UVR of daylight that reaches the earth surface has potentiality to damage biological organisms (Gibson, 2008). Shortwave radiations of different wavelengths are not equally penetrable to ozonosphere and not equally harmful to individuals. UVR is divided into UVA (wavelengths between 315 - 400 nm), UVB (wavelengths between 280 - 315 nm) and UVC (wavelengths between 200 - 280 nm). UVA is responsible for premature aging,
30 skin wrinkling and even skin cancer and can fully penetrate through ozonosphere. UVB is more risky than UVA, but less dangerous than UVC. UVB can cause cataracts, sunburns and skin cancers to human. UVB is partially absorbed by ozonosphere. UVC is extremely dangerous, but completely absorbed by ozonosphere, and cannot reach the earth surface (Gibson, 2008; MacDonald et al., 2006). As a result, the most critical part of UVR is UVB which is partially absorbed by ozonosphere and has a possibility to increase in the future due to the impact of climate change (discussed latter in Section 2.5). Figure 2.1 shows the spectrum of solar radiation with classification of UVR, and summaries the findings relating to the major negative health effects of prolong exposure to UVR. The major negative health effects of prolong exposure to UVR are described below.
Wavelength in Nanometers
0 100 200 300 400 500 600 700 800 900 1000 1100
Gama & X-ray UVR Visible spectrum Infrared
200 280 315 400
UVC UVB UVA
More risky than UVA, but less dangerous than UVC
Can cause cataracts, sunburns and skin cancers to human.
UVB is partially absorbed by
ozonosphere
Responsible for premature
aging, skin wrinkling and even skin cancer.
UVA can fully penetrate
through ozonosphere
Extremely dangerous
But absorbed by ozonosphere, and cannot reach the earth’s surface.
O z o n o s p h e r e
Figure 2.1: Distribution of UVR and summary of the findings about major health effects of exposure to UVR (adapted from: Gibson, 2008; MacDonald et al., 2006).
a. Immune suppression
There is a possibility that UVB exposure can cause suppression of the immune response to animal and human body (Kovats, 2008; Longstreth et al., 1998). Human infectious diseases have shown an effect of UVB exposure in animal models for herpes, tuberculosis, leprosy, trichinella, candidiasis, leishmaniasis, listeriosis and lyme disease (HPA, 2002). UVB can also activate viruses such as herpes, HIV and human papilloma and could adversely affect the course of some infectious diseases in humans as well as
31 the effectiveness of some vaccinations (Kovats, 2008). UVB exposure can reactivate latent infections (Rooney et al., 1991), and with induced immune suppression may cause some cancers, such as squamous cell skin cancer. Evidence support that the incidence of Non-Hodgkin‟s Lymphoma (NHL) shows a positive association with UVB levels in most developed countries including England and Wales (Bentham, 1996) and worldwide. Studies from the USA do not show the same association (Freedman et al., 1997) and daylight exposure could exacerbate HIV infections were not supported in a USA study (Saah et al., 1997). Epidemiological evidence of immune suppression on the potential impact on human health remains sparse and insufficient (Longstreth et al., 1998; de Gruijl, 1997) and researchers are accumulating information on the mechanisms by which exposure to UVR causes immune suppression, but direct evidence on what the implications are for human health is still indefinable (UNEP, 2003).
b. Breast cancer
Studies have found a potential link between light pollution and hormone production, specifically related to melatonin and estrogen levels in women (Coyle, 2004). The presence of light exposure at night time reduces melatonin levels, which elevate estrogen levels in women who did not sleep at night often, and increases responsiveness of estrogen-dependent tissues to cellular proliferation. As a result, the risk of breast cancer increased (Davis et al., 2001). Schernhammer et al. (2001) conducted a 10 years follow-up study on nurse‟s health study cohort and revealed that breast cancer risk increased moderately among female nurses who frequently work in rotating night shifts.
c. Skin cancers
As the depth of penetration of UVB is very short, skin and eyes of human body are more in risk to damage by UVB exposures. Studies confirmed that increased UVB exposures are expected to raise skin cancers (Kovats, 2008; UNEP, 2003). There is strong evidence that exposure to UVB is a major aetiological factor for both non- melanoma skin cancers (NMSC) and malignant melanoma (MM) (HPA, 2002). The different types of skin cancers show important differences in the relationship between solar exposures and risk levels (Longstreth, et. al., 1998).
32 Increased temperature may also enhance the carcinogenic potential of exposure to daylight, although the evidence is speculative. Study estimates that a 2°C increase in ambient temperature might result in 21% increase in the incidence of skin cancer, which is substantially greater than any anticipated effects of ozone depletion alone. This estimation is based on extrapolation from the results of experiments on mice and there is, yet, no direct evidence for humans (Kovats, 2008).
d. Eye damage
Exposure to daylight is associated with a variety of eye disorders. Among them, the most significant one from a public health perspective is cataract. The lens affected by cataract gradually loss its transparency to frequently blindness. The treatment is to replace the affected lens by surgery. Several epidemiological studies have shown an association between cortical cataract incidence and individual UV exposure levels (Taylor et al., 1988). There is uncertainty about which part of the solar spectrum (UVA or UVB) is responsible for cataract (de Gruijl, 1997). As ozone depletion would affect UVB levels but have little influence on UVA, the doubts about the action spectrum has made it difficult to estimate the effects of UVR on cataract incidences (HPA, 2002). Although there were some uncertainties remained about the role of daylight exposure in the formation of cataract, new studies from Australia (Neale et al., 2003), France (Delcourt et al., 2000) and a review of 22 published studies (McCarty and Taylor, 2002), supported an association between exposure of daylight and cataract with animal models (UNEP, 2003) particularly implicating UVB.
e. Sunburn
Prolonged exposure to UVR will turn skin either brown (a suntan) or red (a sunburn) and over prolonged exposure will break chemical bonds of skin tissue, may cause skin wrinkle. Sunburn is the most obvious effect of exposure to UVR from the sun (erythema) (Kovats, 2008). Over exposure of the sun can cause pain and blister to skin and may take several days to resolve. Severity depends on the intensity and duration of exposure. A general response to UVB exposure is thickening of the skin and in many individuals (depending on skin type), the development of a tan provides some protection (HPA, 2002).
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2.5. Impact of climate change on UVR
It is evident from above discussion that the most of the adverse effects of daylight are associated with exposure to UVR. There is a possibility to increase the adverse impact of daylight due to climate change. The rapidly accelerating climate change, which is mainly associated with GHG emissions, is responsible for many dangerous regional and global environmental events. GHG-related climate change can deplete the stratospheric ozone layer (HPA, 2002). The atmospheric ozone layer acts as a filter against part of short wave radiation (Figure 2.2). As a result, there are possibilities that more downward shortwave radiation will reach to the earth in the future. As, shortwave radiation contains UVR there is a possibility to increase the UVB levels in the future due to the impact of climate change.
O z o n o s p h e r e Indoor workers receive 10-20% of outdoor workers‘ yearly UV exposure (fO) (fc)
Shade can reduce UVR by 50% or more Cloud can reduce UVR
by 10% or more
(ft)
Incident Radiation Incident Radiation
Figure 2.2: Natural elements that affect the transmission of solar radiation to the earth surface (adapted from: Gibson, 2008; CCV, 2004).
There are some natural elements in the environment that affect the transmission of UVB to the earth surface. Figure 2.2 shows different natural elements in the environment, which are responsible for reducing UVB exposures such as ozone layer (fO), clouds (fC)
and trees (ft) (Gibson, 2008). UVB is partially absorbed by Ozonosphere in first
instance. Light cloud can reduce UVB by 10% and heavy cloud can reduce more. Shades and trees can reduce UVB by 50% or more, however, individuals who stay
34 inside indoor environment have a risk to receive 10-20% of UVB radiation in a year through windows and openings, compared to individuals who are engaged in outdoor works (CCV, 2004) (Figure 2.2).
Rapidly accelerating climate change may also cause decrease of cloud cover and reduction of green. The decrease of cloud cover is proportional to the increase of UVR levels in environment for example if the cloud cover decreases by 4%, the ambient UVR levels can be expected to be increased by ~2% (Diffey et al., 1994). According to United Kingdom Climate projection 2009 (UKCP09), the changes in mean cloud amount during summer can be decreased up to – 18% (-33 to - 2%) in some parts of UK (southern) which will result an addition of + 16 W/m2 (-2 to + 37 W/m2) in downward shortwave radiation over the 21st century (Jenkins et al., 2009). Therefore, it can be assumed that the amount of incident UVR on earth will be increased in the future. Longer summers and permanent changes in cloud cover may lead to changes in the levels of personal exposure to UVR both outside and inside of the buildings. Therefore, 10-20% of outdoor UVR received by indoor occupants can be a threat for some particular geographical location in particular periods of the year.
The amount of UV exposure depends on the geographical location of the place (altitude and latitude) for example a country located in the southern hemisphere is closer to the sun in summer due to the earth‟s oval shaped orbit and will receive more UVR during summer compared to a country located in similar latitudes in the northern hemisphere. On the other hand, the depletion of ozone layer due to climate change is not uniform over the globe. As a result changes in UV-levels sometimes vary significantly under same hemisphere between two adjacent locations. Slaper et al. (1998) estimates location specific changes in UV-levels for European countries by using satellite data on ozone depletion. Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) was used to measure ozone, and UV-transfer model (Slaper et al., 1992) was used to estimate changes in ground level (Bordewijk et al., 1997) over the period 1980 to 1991 (Figure 2.3). Figure 2.4 shows the changes in UV radiation in Europe over 1980 to 2000 (EDC, 2000). From Figure 2.3 and 2.4, it is evident that relative changes in UV level were largest (8%) in north-west Europe considering 10 years from 1980 and in Central Europe (7-8%) considering 20 years among European countries. If the increase ratio of Figure 2.3 continues over a life time, the excess skin cancer risk at 520 north latitude are shown in
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