UV radiation at work and health

Introduction

Exposure to UVR can induce both acute and chronic adverse health effects. This concerns both outdoor workers exposed to solar UVR and indoor workers exposed to artificial sources of UVR. The occupational health risks from artificial UVR sources can be more significant than from solar UVR, because UVR levels may be higher and may include short wavelengths, which are normally filtered by the Earth’s atmosphere.

UVR effects on the skin

UVR health effects in general

UVR may have positive as well negative effects on human health, depending on the conditions of exposure and wavelength of radiation. While excessive UVR exposure can cause adverse photochemical reactions, DNA lesions, damage to the skin and the eyes and can affect the immune system, small amounts of UV are beneficial for people, in fact essential for the production of vitamin D. Further benefits of UV are found in medicine where UV lamps are used for medical treatments for example of many dermatological diseases like psoriasis and eczema, as well as jaundice in new-born babies. Such applications are performed under medical supervision and the benefits of treatment versus the risks of UVR exposure are a matter of clinical judgment.

Structure of the skin

The human skin is constituted by three main layers: the epidermis, the dermis and the subcutaneous tissue. The epidermis is formed by a basal layer of dividing keratinocytes. Within the basal layer, melanocytes from neural origin are inserted between basal keratinocytes, their dendrites being in contact with as many as 50 keratinocytes, forming the melanin unit. Subcutaneous tissues are constituted by fat-containing cells. This layer constitutes a mechanical shock absorber between the surface of the skin and the deepest muscles and bones. Nerve endings are located immediately under the epidermis and are responsible for the feeling of pain after UVR overexposure^[1].

Acute effects on the skin

Erythema

Erythema (“skin burn"), an acute injury following excessive exposure to UV radiation is the result of a phototoxic (actinic) effect in the skin. Unlike other burns, UVR erythema is not immediate. Erythema (red appearance of the skin) results from an increased blood content near the skin’s surface and reaches maximum at about 8-12 hours after exposure, fading to normal within a few days. In outdoor work, the non-adapted (“untanned") skin of very lightly pigmented subjects will normally experience mild reddening after about four hours following only a half-hour exposure to midday summer sunshine in southern Europe. Higher doses may result in pain and skin swelling (oedema) with blistering, and after a few days, peeling. Sunburn sensitivity varies substantially with skin complexion and colour, and this is reflected in the solar exposure time required to induce a sunburn reaction - from 15-30 minutes of midday summer sunshine to 1-2 hours exposure for moderately pigmented skin; and those with darkly pigmented skin may not clearly perceive sunburn for a full day’s exposure. Skin specialists frequently group individuals into one of six sun-reactive skin types, and these skin types fall into three more significant groups based (Table 1) upon how well individuals produce the melanin pigment in their skin^{[1] [2][3]}.

Specialised measurement quantities are useful when describing sunburn sensitivity. A person’s Minimal Erythemal Dose (MED) is defined as the UVR exposure that will produce a just perceptible erythema 8-24 hours after irradiation of the skin. It is very important to recognise that the MED is individual and it varies with the source of UVR, the tanning capacity and any adaptation from previous exposures. Because the MED measure refers only to individual sensitivity, there exists a related, standardised quantity for source measurement: the Standard Erythemal Dose (SED) to quantify the ability of a source to produce erythema. This unit is widely used in dermatology to measure erythemally effective irradiances. Both quantities are standardised by the International Commission on Illumination (CIE) and the International Standards Organization (ISO)^[2].

Tanning and thickening of the skin

Skin darkening (“tanning") and skin thickening by frequent UVR exposure are obvious consequences of skin adaptation to UVR. Thickening of the outermost layers of the skin (epidermis and stratum corneum) takes place as an adaptation to UVB-related damage. This can be a 3 to 5-fold thickening of the stratum corneum within one to seven weeks after several exposures to UVB, and it returns to normal about one to two months after the exposure has stopped. This thickening after sun exposure leads to a significant increase in UV protection by a factor of five or greater^[4] , and in lightly pigmented skin types, thickening is probably more important than tanning in providing protection. The thickening of the skin after prolonged tanning protects sensitive cells (basal keratinocytes, melanocytes) by absorbing UVB radiation before they reach the basal layer of the epidermis. After some shedding (peeling) of the stratum corneum, the basal layer can be directly stimulated by UVB and thus the thickening or protective processes recur and reach a steady state. However, in darkly pigmented individuals, it is likely that skin pigmentation is the most important means of protection against UVR^[4].

Tanning becomes noticeable within a day or two after sun exposure, gradually increasing for several days and persisting for a week. Although a tanned skin does confer a degree of protection, this seems to be no more than a factor of two to three in the absence of skin thickening^[4]. The wavelengths of the radiation that induce tanning are very similar to those of radiation producing erythema. Table 1 describes the range of skin types and sensitivity to UVR effects. Subjects with sun-reactive, melano-compromised (skin types I and II) are poor tanners compared to those with melano-competent (skin types III and IV) who tan easily^[2] [4].

Table 1: Classification of skin phototypes based on their susceptibility to burn in solar radiation and their ability to tan. NB! The ranges of SEDs are only indicative.

Source: Adapted from ^[6] by the author

Chronic effects on the skin

Skin aging

Cutaneous skin aging (photo-aging) from occupational exposure has traditionally been particularly observed in fishermen and farmers in sun exposed sites such as the face and the back of the neck and hands. The clinical signs of a photo-aged skin are dryness, deep wrinkles, accentuated skin furrows, sagging, loss of elasticity, mottled pigmentation and the development of tiny but highly visible, superficial blood vessels. These characteristics reflect the profound structural changes occurring in the dermis. It is not clear which wavelengths are most responsible for skin aging, but most studies point to solar UVA exposures as significant factors^{[1] [4]}.

Skin cancers

The three common forms of skin cancer are: basal cell carcinoma (BCC), squamous cell carcinoma (SCC) and malignant melanoma (MM). Around 90% of skin cancer cases are the non-melanoma types (BCC and SCC) with BCCs being much more common than SCCs. Exposure to UV radiation is considered to be a major etiological factor for all three forms of cancer^{[3] [4]}.

Within the framework of the WHO/ILO Joint Estimates of the Work-related Burden of Disease and Injury a systematic literature study (2021)^[5] was carried out to review the evidence for the relationship between occupational exposure to solar UVR and the incidence of malignant skin melanoma and non-melanoma skin cancer. The review study concluded that for the health outcome of melanoma incidence, the strength of evidence can be judged as “limited evidence for harmfulness”. For the health outcome of non-melanoma skin cancer incidence, the strength of evidence was judged as “sufficient evidence of harmfulness”. This means that for both groups of cancers - malignant skin melanoma and non-melanoma skin cancer – outdoor workers are at increased risk. 

Traditionally, non-melanoma skin cancer (NMSC) has been considered a tumour typically seen in elderly, male farmers, but in epidemiological studies this has not always been confirmed. A possible explanation is that people with a tendency to sunburn are under-represented amongst outdoor workers, suggesting that there may be a self-selection of sun-tolerant individuals to become outdoor workers. Furthermore, outdoor workers are more likely than others to wear hats and other PPE for sun protection^[6].

Cutaneous malignant melanomas (CMMs) are responsible for 80% of skin cancer deaths, even though CMMs represent only 5% of the skin cancer cases. Both intermittent recreational exposures and cumulative occupational UVR exposures are etiological factors for CMMs. Taking into account that melanoma is one of the tumours whose incidence is increasing most, there is a need to take appropriate actions to improve policies and practices related to minimising the outdoor workers’ exposure to solar radiation^[7] .

However, indoor workers are not spared of these types of cancers. Analyses of data from large studies have generally found greater risk of melanoma in indoor than outdoor occupations, although the reverse has been found for head and neck melanoma. In individual-based studies, the results of risks to outdoor versus indoor workers have been contradictory^[8][9].

UVR effects on the eye

Structure of the eye

Anatomically, the eyes are protected against UV radiation from most directions, despite from UV incident directly or from the side. However in occupational situations, an artificial UVR source is very frequently within the normal direct field-of-view of the worker, thus permitting direct exposure of the eye. Exposure of the eye to UVR is associated with a variety of disorders, including damage to the eyelids, cornea, lens and retina.

The UV radiation reaching internal structures of the eye is attenuated depending upon the wavelength of incident radiation (Figure 1).

Figure 1: Penetration of UV radiation into the eye.

Source: Adapted from^[10]by the author 

Radiation with wavelengths shorter than 290 nm (UVC) is almost entirely absorbed by the cornea. Further, radiation in the range 300-370 nm is almost entirely attenuated in the lens. There is a strong increase of UVR attenuation by the lens with increasing age. If the lens is removed (cataract surgery) without implantation of an UVR absorbing lens, a significant amount of the incident UVR (290 - 400 nm) may reach the retina. The cornea does not adapt to repeated exposures like the skin which reacts by thickening of the stratum corneum and epidermis. Therefore, the cornea is equally vulnerable day after day to the same exposure to UVR.

Table 2: Summary of the main adverse effects on the eyes of the different wavelength bands of UVR

Source: Adapted from^[11]by the author

Acute UV-related eye diseases are: solar rethinopathy, photokeratitis and conjunctivitis. In the group of chronic outcomes are: pterygium, squamous cell carcinoma of the cornea, squamous cell carcinoma of the conjunctiva, cataract, ocular melanoma and macular degeneration. Further data are required to clarify the relationship between excessive UVR exposure and acute macular degeneration, nuclear and posterior subcapsular cataract. Only few studies have examined occupational risk factors for chronic macular degeneration but data suggest that long-term occupational exposure to solar UVR, in particular for its blue-light component, is associated with macular degeneration in outdoor workers^[12]. There is limited evidence of an association between UV-B exposure and ocular melanoma. These melanomas include both external (eyelid and conjunctiva) and intraocular (iris, ciliary body and choroid) tumours, but epidemiological data suggest that UVR plays a role only in external tumours^[13].

Acute effects

In outdoor work, unprotected eyes which are exposed to solar UVR for one day may accumulate a sufficient dose to cause an adverse effect in the cornea of the eye. As with sunburn of the skin, the symptoms are delayed for several hours. Most welders have experienced similar symptoms, which is popularly referred to as “welder's flash" (or “sand in the eyes")^[14] .

Within six hours, this kind of UV exposure gives rise to feeling of itchiness, increased lacrimation (secretion of tears), severe pain and photophobia (light sensitivity). This is attributable to an inflammatory reaction in the cornea and conjunctiva, which leads to a swelling and loss of the superficial cells in the cornea and the conjunctiva. The pain decreases and the light sensitivity disappears within a couple of days^[15] .

In addition to corneal injury, laboratory studies have demonstrated acute cataract formation from UVR at wavelengths greater than 310 nm emitted by artificial or laser UV sources^[16][17]. In the unusual situation where the UVR absorbing lens or lens implant is not present, a retinal injury is possible for wavelengths greater than approximately 300 nm^[10].

Chronic effects

Development of cataract, clouding of the lens that disturbs vision, is part of the natural ageing process. There is epidemiological data showing an increased risk for cortical cataract with UVB exposure from the sun^[11]. Worldwide at least 2.2 billion people have a near or distance vision impairment and in at least 1 billion of these cases, vision impairment could have been prevented or still needs to be addressed. Among these 1 billion cases, the main cause is cataract (94 million)^[18].

Pterygium is a fibrous ingrowth of the cornea of tissue making the cornea opaque. Chronic exposure to UV radiation has been shown to be related to pterygium^[19][20].

Epidemiological studies of the relationship between UV exposure and several age-related ocular diseases, such as pterygium, cataract, and ocular melanoma, have been carried out by several research groups. Cross-sectional studies have shown an association between chronic high levels of outdoor UV radiation exposure and risk of cortical cataract^[21] .

Case-control studies have shown an elevated risk of ocular melanoma in subjects with light skin, hair and eyes, and with many cutaneous naevi^[22][23]. These studies have given limited support to association of eye melanoma with exposure to artificial UV sources, such as electric arc welding.

UVR immune effects

There is strong evidence that UV radiation affects immunity and hence can aggravate infections and allow the development of skin cancers. It has been established that UVB exposure reduces specific immune reactions in the skin of healthy people. This effect is usually only temporary, but it can have clear drawbacks when it coincides with an infection or with abnormal cell growth.

UVR can alter various organic molecules in the skin through photochemical reactions, so that these molecules become dysfunctional to the skin. An important task of the body’s immune system is to seek out and destroy any “foreign intruders," and it is therefore conceivable that UV radiation could trigger unwanted immune reactions against the skin (‘sun allergy’)^[24].

Not only infections in the skin can be aggravated, but also internal infections may be exacerbated. The common ‘cold sore’ that is evoked by sun exposure is an example of an infection (by stimulating the Herpes simplex virus) in humans caused by UVB exposure^[1].

UVR and photosensitizing agents

Some chemicals increase the sensitivity of human skin to UVR and produce phototoxic reactions, i.e. cutaneous reactions with rash, erythema, itching, intense sunburn, blistering or oedema^[1]. Such substances are called photosensitizers. They absorb optical radiation (generally UVR) and transfer the energy to reactive biomolecules that can produce a toxic reaction at doses well below those that induce for example sunburn or keratitis. These photosensitizing effects can occur in the case that these individuals are exposed to UVR in the frame of their work.

There are many different types of photosensitizers e.g. certain drugs, plant materials, perfumes and cosmetic constituents, dyestuffs, polycyclic hydrocarbons in wood preservatives, coal tar pitch and petroleum products containing polycyclic aromatic hydrocarbons (PAHs), sunscreen, and certain printing chemicals used in photosensitive printing processes. There are many photosensitizing drugs e.g. thiazide diuretics, drugs used in the treatment of high blood pressure, certain antibiotics (tetracyclines, sulfonamides), oral contraceptives, and thiazine tranquilizers (e.g. chlorpromazine, phenothiazine anti-depressant drug) These drugs must be taken internally before the skin becomes sensitive to sunlight. However, simple skin contact with various plants and fruits, such as figs, parsnips, citrus plants, carrot, dill, mouldy celery and some types of weeds, followed by exposure to sunlight can cause a phototoxic reaction such as sunburn or dermatitis. Citrus fruit handlers and vegetable harvesters, gardeners, florists and bar tenders are at risk. Some individuals who have been exposed to photosensitizers and have experienced a phototoxic reaction may experience permanent skin reactions when exposed only to the sun.

Some photosensitizers are also photo-allergens^[1]. While phototoxic reactions are usually localised to the body surface at the site of exposure, the effect of a photo-allergic reaction extends beyond the site of exposure. A further difference is that in most cases a phototoxic reaction is proportional to the concentration of the photosensitizer and to the magnitude of UV exposure, whereas a photo-allergic reaction depends on the amplitude of the immunologic reaction.

According to Directive 2006/25/EC^[25], the employer should give particular attention, when carrying out the risk assessment, to […] any possible effects on workers’ health and safety resulting from workplace interactions between optical radiation and photosensitizing chemical substances.

In addition, some chemicals called "promoters" can increase the cancer-causing ability of UVR. And vice-versa, UVR itself can act as a promoter and increase the cancer-causing ability of some chemicals, in particular from coal tar and pitch^[26] .

UVR as occupational disease

The European schedule of occupational diseases^[27] exclusively includes ‘conjunctival ailments following exposure to ultraviolet radiation’ (No 502.02) but no UV-generated cancer. The EU Member States are encouraged to develop their approach on occupational diseases based on the European schedule but each Member State has its own legislation and lists of recognised occupational diseases. In many EU countries, no diseases related to UVR exposure are on the list of occupational diseases and in almost all countries occupational cancers due to UVR do not fall under the compensation system on occupational diseases. An exception is Germany where UV-induced skin cancer is included in the list of occupational diseases since 2015. With 9905 cases reported in 2018, work-related skin cancer is already the third most reported and the second most recognised occupational disease, and by far the most common work-related cancer in Germany^[28].

Health surveillance and monitoring

The European Directive 2006/25/EC, concerning the minimum health and safety requirements regarding the exposure of workers to risks arising from artificial optical radiation^[25], describes the requirements for health surveillance of workers exposed to artificial UVR, based on the Council Directive 89/391/EEC on the introduction of measures to encourage improvements in the safety and health of workers at work^[29].

Health monitoring provides baseline to detect adverse health effects in workers exposed to UVR. Health monitoring is effective for the individual worker, if useful screening techniques (medical exams) are available to identify abnormalities in the target organ system. Consultation with the industrial hygiene staff is strongly recommended. This kind of collaboration gives crucial information on the actual UVR exposure history, needed for accurate diagnosis of the worker’s health problems.

Medical examination should be made available to workers if it is suspected or known that they have been exposed to artificial UVR in excess of the exposure limit value. In addition, a medical examination should be performed if a worker is found to have an identifiable disease or adverse health effects, which is considered to be a result of exposure to artificial UVR. It is important that the person carrying out the medical examination is familiar with the potential adverse health effects from the specific sources of workplace exposure to artificial UV radiation.

Health surveillance should be carried by an occupational health professional, or a medical authority responsible for health surveillance in accordance with national law and practice. EU Member States are responsible for establishing arrangements to ensure that individual records exist and are kept up to date. The records should contain a summary of the results of the health surveillance. The records should be in a form which allows them to be consulted afterwards, taking account of confidentiality. Individual workers shall have access to their own records on request.

If the exposure limits have been exceeded UVR exposure limits or if the adverse health effect is considered to have been caused by UVR exposure in the workplace, the following actions shall be carried out^[30] 

• The worker should be informed of the results;
• The worker should receive information and advice regarding follow-up health surveillance;
• The employer should be informed, respecting any medical confidentiality;
• The employer should review the risk assessment;
• The employer should review the existing engineering and administrative control measures;
• The employer should arrange any necessary continued health surveillance of the exposed worker.