Solar Radiation is the radiant energy, particularly electromagnetic energy, emitted by the sun. About half of this radiant energy is in the visible part of the electromagnetic spectrum whilst the other half is comprised mainly of near-infrared (IR) and some ultraviolet (UV) portions of the spectrum. Solar radiation is the energy source that drives the various systems (climate systems, ecosystems, hydrologic systems, etc) on Earth and is a requirement for life as we know it. However, over exposure to the sun's radiant energy, in particular the shorter wavelength, higher energy UV light, can cause detrimental health effects.
This article examines the effects of solar radiation on the human population with focus on flight crews.
Solar energy is created in the core of the sun when hydrogen atoms are fused into helium by nuclear fusion. For each second of the solar nuclear fusion process, millions of tons of hydrogen are converted into the heavier atom helium. The solar nuclear process also creates immense heat that causes atoms to discharge photons which are the elementary particles of light and radiation. Photons are emitted and then reabsorbed by adjacent atoms numerous times over thousands of years as they slowly migrate from the core to the surface of the sun. From the surface, the photon's journey to earth takes approximately 8 minutes.
The photons, or energy, emitted by the sun pass through space until they are intercepted by some object, be it a planet, another celestial or man-made object or by dust or gas. The intensity of the radiation striking the object is determined by the inverse square of the distance from the source. That is, if the intensity of radiation at a given distance is one unit, at twice the distance it will be one-quarter the original intensity and at three times the distance one-ninth the original intensity. The sun to earth distance of approximately 150 million kilometers diminishes the radiation level at the outer reaches of the earth's atmosphere by a factor of about 45,000 compared to that found at the surface of the sun.
As the radiation passes through the atmosphere, some of the visible radiation is reflected and scattered. Approximately 30% of the sun's visible radiation is reflected back into space by the atmosphere or by the earth's surface. Much of the short wavelength, high energy radiation such as gamma rays, X-rays and UV rays are selectively absorbed by nitrogen, oxygen and ozone. Longer wavelength radiation such as infrared is partially absorbed by ozone, carbon dioxide and by water present in the atmosphere in both liquid and vapour form. As a result of all of these factors, only about 40% of the radiation at the outer reaches of the atmosphere actually reaches the surface of the earth at sea level.
Although the amount of radiation leaving the sun is dramatically diminished by distance and atmospheric filtering, there is still sufficient energy in the ultraviolet (UV) rays that reach the surface of the earth to have adverse effects on the human population. the effects can be more pronounced in equatorial regions where the sun's rays are more intense and at higher altitudes where there is less atmospheric filtering of the UV rays. Fair-skinned people are more susceptible to the effects of UV rays as are those under 6 or over 60 years of age. These effects include sunburn and photokeratitis (snow blindness) and cumulative exposure can increase the risk of some cancers.
Sunburn is a type of radiation burn which affects living tissue, mainly skin. It results from an overexposure to UV radiation, most commonly from the sun but potentially from other sources such as a tanning bed/lamp or welding arc. The severity of the burn can range from minor to extreme depending upon the duration of the exposure, the intensity of the radiation, the amount of pigmentation in the individual's skin and any protective measures taken. Severe sunburn can be debilitating and may require hospitalization. Sunburn is the body's reaction to the direct DNA damage to affected skin cells caused by exposure to UV radiation.
Sunburn of unprotected skin can occur after as little as 15 minutes exposure to the sun and far more quickly when exposed to more intense UV sources. Symptoms of sunburn include an initial reddening of the skin which can occur in as little as 30 minutes but more typically in 2 to 6 hours. The redness is followed by pain which is proportional to the severity of the burn and is most intense 6 to 48 hours after the burn has occurred. Sunburn can be accompanied by dizziness and nausea and, in severe cases, blisters may form. Sunburn also increases sensitivity to heat and may cause chills or fever. Treatment can include avoidance of further exposure to the UV source, pain killers, topical creams and moisturizers.
The body recognizes the DNA damage caused by the UV radiation and takes steps to repair or eliminate the damaged cells. This can result in the drying and peeling of the affected skin areas over the days following the burn occurrence. The repair mechanism also results in the production of melanin which darkens the skin (tanning) and acts as a photo-protectant to help reduce the effects of further UV exposure.
Photokeratitis is also referred to as ultraviolet keratitis, snow blindness or flash burn. It is a painful eye condition which is caused by the exposure of the eyes to UV radiation from either a natural or artificial source. For all intents, photokeratitis is a sunburn of parts of the eye, specifically, the cornea and mucous membranes or conjunctiva. Reflected UV glare from snow, ice, water or cloud can exacerbate the potential for the condition in natural light, especially at higher altitudes.
Like sunburn, photokeratitis in not usually apparent until several hours after the exposure. Symptoms include extreme light sensitivity, increased tearing and pain, often intense, which is likened to having sand or grit in the eyes. Treatment usually includes removal from the light source, administering eye drops and pain killers and covering the eyes with cool, wet compresses. Healing is normally rapid (24-72 hours) if there is no further exposure to the UV source.
Skin cancer is the most common form of cancer with occurrence totaling more than half of all cancer cases. There are three primary types of skin cancer: basal-cell cancer (BCC) and squamous-cell cancer (SCC), referred to non-melanoma cancers, and melanoma. Non-melanoma cancers are relatively easy to treat, have a fairly low metastasis frequency and rarely result in death. Melanoma, on the other hand, can be a very aggressive cancer, is likely to metastasize or spread throughout the body, is more difficult to treat and has a fairly high mortality rate.
There are several factors which increase the likelihood of contracting a skin cancer. The most significant of these is exposure to UV radiation. Skin cancer is most often caused by years of too much sun exposure. More than 90% of all skin cancers are found on body parts that get the most sun most of the time. Contributing factors include:
- Pale skin - individuals with little skin pigmentation (usually red or blonde hair) that sunburn easily
- A history of severe sunburns
- Family history of cancer
- Older age
- Weakened immune system
Relevance to Air Crew
Flight and Cabin Crew are no more susceptible to the effects of solar radiation than are most members of the general population. However, aspects of the profession, inclusive of increased exposure to high altitude and the increased potential for frequent short term ground level sun exposure during layovers due to a long range flight schedule, can put them at higher risk for sunburn, in some environments, photokeratitis and possibly increase their cancer risk.
It is not unusual for a crew, whose home base is in the mid-latitudes, to be assigned to a flight which results in a layover in the tropics. Wanting to take full advantage of the short respite from the winter weather at home, members of the crew often spend the full day at the beach, by the pool or just out in the sun, often without taking appropriate measures to protect themselves from a level of UV radiation that is much more intense than that to which they are accustomed. At departure, the resulting sunburn can make the wearing of a uniform uncomfortable, the donning of a seatbelt and harness painful and the severe sunburn effects of fever, chills and dehydration potentially debilitating. The safety of flight risk due to incapacitation has been increased significantly due to what is essentially a self-inflicted injury.
Photokeratitis from natural UV light sources is generally caused by the reflection of sunlight from water, sand or, most usually, snow. A pilot supporting Arctic operations such as mineral exploration, scientific research or even a hunting party might well spend a number of hours on the ground waiting for his passengers. Given the potentially remote circumstances of the landing site, this time might well be spent out-of-doors and, if the sky is clear and the sun shining on fresh snow, the potential for snow blindness can be significant if appropriate protective measures are not taken. As the onset of photokeratitis is fairly sudden and does not occur for several hours after exposure, the risk of total incapacitation on the return flight may be increased. In a single pilot fixed wing or helicopter operation, this incapacitation could easily result in Loss of Control.
UV and other radiations are filtered by the earth's atmosphere and less than half of the radiation present at the outer reaches of the atmosphere reaches the earth's surface at sea level. At altitudes greater than sea level, higher radiation levels are present. Whilst aircraft windows and structure block the bulk of the ambient radiation, there is a level of exposure to crew and passengers on any high altitude flight. Flight crew have a proportionally higher exposure than the rest of the crew and passengers due to the larger window area in the cockpit. Flight frequency results in a cumulative exposure rate to air crew which exceeds the norms of the general population. Although a direct link has not been proven, this cumulative exposure could potentially lead to an increased cancer risk.
The primary prevention measure for all three UV radiation induced health issues is a combination of avoidance and protection. Prevention strategies include:
- Apply sunscreen with an appropriate SPF (Sun Protection Factor) before sun exposure
- Minimise sun exposure to the extent practical
- Minimise exposure during peak UV index periods
- Decrease or discontinue use of tanning beds/lights
- Wear protective clothing - long sleeves, long trousers and a hat
- Wear protective eyewear - sunglasses, snow goggles, welding goggles etc as appropriate to the situation and UV source