Formaldehyde toxicology – occupational versus environmental HAP?
Clean Air Journal
Scientific paper logical data sets to development of health risk estimates and safe exposure levels in the absence of adequate understanding of biological mechanisms of toxicity and dose-response behaviour are illustrated and appropriate limits for occupational and ambient air quality in South Africa are proposed based on latest scientific knowledge. Formaldehyde as an air pollutant Formaldehyde is the most common naturally occurring aldehyde in the environment. It is an essential metabolic
... ntial metabolic intermediate in all cells, both human and animal, and is produced endogenously. Formaldehyde is, however, also generated by a range of anthropogenic sources including automotive exhaust, cigarette smoke and various products containing formaldehyde-based resins and glues used in the manufacture of particle board and plywood. Occupational exposures occur in a wide variety of settings and industries which can be summarised into three main circumstances, namely (i) production of aqueous formaldehyde (formalin) and its use in chemical industry (eg resins, preservative etc.), (ii) in relation to its release from formaldehyde-based resins in which it is present as a residual and/or through the hydrolysis and decomposition by heat namely during manufacture of wood products, textiles, synthetic vitreous insulation and plastics, and (iii) in relation to the pyrolysis or combustion of organic matter, namely engine exhaust gases or during fire fighting. Environmental, or non-occupational, exposures to airborne formaldehyde, range from around 0.5 ug/m3, which represents the mean natural background concentration (WHO, 2001) to up to 100 ug/m3 as short term peak levels recorded in heavy traffic or during severe inversions in urban environments. The levels of formaldehyde in indoor air are often higher than those in outdoor air due to a variety of anthropogenic sources, notably offgassing of urea-formaldehyde foam insulation, particle board and formaldehydebased resins. Mean levels in homes with no urea-formaldehyde foam insulation range from 25 to 60 ug/m3 whereas levels in mobile homes have been recorded over 100 ug/m3 (IARC, 1995). Formaldehyde is formed in the troposphere by photochemical oxidation of many types of organic compounds from both natural and anthropogenic sources. Given the diversity and abundance of formaldehyde precursors in urban air, secondary atmospheric formation frequently exceeds direct emissions from combustion sources, especially during photochemical air pollution episodes. Formaldehyde is not a persistent air pollutant being highly reactive with photochemicaly generated hydroxyl radicals, undergoing direct photolysis, as well as rapid hydrolysis on contact with water vapour. Atmospheric half-life is strongly dependent on intensity of sunlight, temperature and humidity/moisture content but is no longer than a few hours to days in most climatic conditions. As a ubiquitous air pollutant in both indoor and outdoor air the potential adverse health effects from formaldehyde exposure have been subject to numerous critical public health reviews by regulatory agencies. Public health advisories have, however, been based primarily on the linear extrapolation of dose-response data from occupational studies where repeated elevated levels of exposure have led to both respiratory irritation and sensitisation as well as probable increased incidences of nasopharyngeal cancers. In this paper the inadequacies in the interpretation of occupationally derived dose-response relationships to derive safe levels for public exposure to a key HAP in the absence of a detailed understanding of dose-response mechanisms are explored. Issues around the 2004 IARC reclassification of formaldehyde as Class 1 'carcinogenic to humans' are discussed and the technical background as to why public cancer incidence rates from formaldehyde exposure have historically been overestimated by a number of international regulatory agencies. The latest knowledge of formaldehyde toxicology and carcinogenesis is discussed to illustrate the limitations in simple linear extrapolation of occupational data to derive cancer risk estimates and safe exposure limits for assessment of ambient air quality monitoring data and public health risk assessment. Occupational and ambient limits for formaldehyde in South Africa are tabled for review and debate.