Balance between Health Risks and Benefits for Outdoor Workers Exposed to Solar Radiation: An Overview on the Role of Near Infrared Radiation Alone and in Combination with Other Solar Spectral Bands

Abstract

Near infrared or infrared A (IRA) accounts for over 40% of the solar spectrum (SS) and is able to reach subcutaneous tissue as well as the retina. Outdoor workers are occupationally exposed to solar radiation (SR), but the level of exposure may differ widely depending on the job performed, time spent outdoors, latitude, altitude, season, personal protection, etc. Until now, risk assessment and management for outdoor workers has focused on the prevention of both acute and long-term effects on the eye and the skin due to solar ultraviolet radiation (UVR) with little consideration of the other components of the SS (a possible exception is represented by visible radiation with reference to the eye). A growing body of evidence coming from in vitro studies indicates that IRA is involved in cellular reactive oxygen species (ROS) production and may interfere with the respiratory chain in the mitochondria. Moreover, it can modulate gene expression and some metabolic pathways. The biological action of IRA is only partly attributable to a thermal mechanism, should it be also involved in photochemical ones. The cellular and molecular pathways affected by IRA are partly similar and partly different with respect to those involved in the case of visible ultraviolet A (UVA) and ultraviolet B (UVB) radiation. Consequently, the net effect of the SS is very difficult to predict at different levels of the biological organization, making more difficult the final balance of health risk and benefits (for the skin, eye, immune system, blood pressure, etc.) in a given exposure situation. Moreover, few in vivo studies and no epidemiological data are presently available in this regard. Investigating this topic may contribute to better defining the individual exposome. More practically, it is expected to bring benefits to the risk assessment and management for outdoor workers exposed to SS, contributing to: (1) better definition of the individual profiles of susceptibility, (2) more focused preventive and protective measures, (3) better implementation of the health surveillance and (4) a more effective information and training.

Keywords: adverse effects; health surveillance; immune response; near infrared; outdoor workers; preventive and protective measures; solar radiation.

Figure 1

An overview of the health effects due to SR (Solar Radiation). Question marks and dashed arrows indicate effects not yet established or not yet fully established. Most of SR health effects are induced by UVB (UltraViolet B) and UVA (UltraViolet A), but for other ones visible radiation may be directly involved (e.g., circadian regulation, a subset of phototoxic and photoallergic reactions and macular degeneration). The balance between potential beneficial and detrimental effects in a given situation of exposure and for a given individual is often very difficult to address.

Figure 2

A schematic picture showing the ability of different spectral bands of optical radiation to penetrate eye. It is evident that only visible radiation and IRA (InfraRed A) may reach retinal tissue in physiological conditions.

Figure 3

A simplified overview of the biological mechanisms underlying both photobiostimulatory and adverse/pathological effects due to IRA (InfraRed A) exposure, with indication of the possible outcomes. The prevalence of one type of effects over the other one is dependent on spectral composition, total dose, dose-rate, biological target and, possibly, other unknown factors.

Figure 4

The balance between potential adverse and beneficial health outcomes for outdoor workers exposed to SR (Solar Radiation) depends on the whole SS (Solar Spectrum) and, in addition to the implementation of preventive and protective measures, is affected by a variety of environmental and occupational co-exposures which, in turn, may be modulated by CC (Climate Change). An extensive research is needed to address properly these concerns.