Ultraviolet Radiation: Human Exposure and Health Risks

Article excerpt

Introduction

Ultraviolet radiation (UVR) is one portion of the electromagnetic radiation (EMR) spectrum. EMR consists of oscillating electric and magnetic fields that can be propagated both in free space and in matter (1). The main groupings of the EMR spectrum (in order of increasing wavelength) are as follows:

* cosmic and gamma rays,

* X-rays,

* ultraviolet radiation,

* visible radiation,

* infrared radiation,

* radar, and

* radio frequency

Ultraviolet, visible, and infrared radiation are collectively known as optical radiation because these wavelengths have effects on the eye. A number of schemes are used to divide the optical radiation section of the EMR spectrum. A frequently used photobiological scheme classifies UVR into three divisions:

1. UVC = 100 to 280 nanometers (nm),

2. UVB = 280 to 315 mm, and

3. UVA = 315 to 400 mm.

The interaction of EMR with matter takes the form of absorption, transmission, reflection, refraction, and diffraction. In most cases, one of these effects will dominate. Each effect is, however, always present to some extent (1). Energy can produce an effect within matter only when it is absorbed. When non-ionizing radiation (such as UVR) is absorbed by a molecule, either it affects the electronic energy levels of the atoms in the molecule, or it changes the rotational, vibrational, and transitional energies of the molecule. In biological systems, this energy transfer produces electron excitation, which can result in dissociation of the molecule, dissipation of the excitation energy in the form of fluorescence or phosphorescence, formation of free radicals (i.e., photochemical injury), and degradation into heat (i.e., thermal injury) (2).

Ultraviolet radiation and other forms of EMR are emitted by many sources and are primarily produced by the following processes:

* incandescence,

* electrical/gaseous discharge such as in arc welding), and

* lasers (3).

The major source of UVR at the earth's surface is the sun, which is an example of an incandescent source. The wavelengths and relative intensities of solar radiation reaching the surface of the earth are affected by a number of factors, including absorption, scattering, and reflection. Ozone, which is found in the stratosphere, has a peak concentration between an altitude of 20 and 30 kilometers (kin). Its absorption band is centered on 250 nm and extends to 350 nm. Ozone thus effectively eliminates all UVC radiation and about half of the UVB radiation from reaching the earth's surface (4). Other meteorological factors that contribute to the attenuation of UVR include the presence of cloud cover, air pollution, haze, and scattered clouds (5).

The aim of this article is to provide an overview of human exposure to UVR and the associated health effects, as well as to present risk estimates for acute and chronic conditions that may result from UVR exposure. The substantial reduction in health risk that can be achieved through preventive actions will also be demonstrated.

Health Effects

Because of the non-ionizing nature of UVR, its interaction with animals - humans in particular - is limited to the skin and eyes. The type and extent of the damage that radiation does to the eye depends on the energy absorbed, the wavelength of radiation, and the duration of exposure (6). When exposed to optical radiation, the various media of the eye act as a series of filters, each component absorbing certain wavelengths to varying degrees (7). A schematic representation of the UVR absorption characteristics of the human eye is provided in Figure 1.

The complex structure of the skin and the presence of structures such as hair follicles, sweat glands, and sebaceous glands make it difficult to determine the exact path that optical radiation travels within the tissue. The presence of optically absorbing molecules (pigments) also affects the penetration of different wavelengths in the skin. …