|Document "Bitumen (vapour and aerosol); Health-based recommended occupational exposure limit": English Abstract|
At request of the Minister of Social Affairs and Employment, the Health Council of the Netherlands recommends health-based occupational exposure limits for the concentration of toxic substances in air in the workplace. These recommendations are made by the Council’s Dutch Expert Committee on Occupational Standards (DECOS). In this report, the committee discusses the consequences of occupational exposure to bitumen (vapour and aerosol). The committee’s conclusions are based on scientific publications before January 2007.
Physical and chemical properties
Bitumen is a black or brown solid or viscous liquid that is obtained from non-destructive vacuum distillation of crude petroleum oil. It is a complex mixture of naphtenic, aliphatic and/or aromatic hydrocarbons and heterocyclic compounds containing sulphur, nitrogen and oxygen. The exact chemical composition, and thus the physical properties, depends on: the crude source; the refinery processes; and, application practices. The refinery process is geared to the desired application.
Bitumen is durable, insoluble in water, and forms strong cohesive mixtures with mineral aggregates. Most of the bitumen is used as a binder in asphalt for road paving and roofing. It is also used in paint to protect surfaces against water. Bitumen is in most cases heated (112-162×C, paving bitumen; 166-229×C, roofing bitumen) during which bitumen vapours and aerosols are released that may contain carcinogenic polycyclic aromatic hydrocarbons (PAH) and compounds (PAC).
Environmental and biological monitoring
Various methods are used to monitor bitumen-derived vapours and aerosols. The more conventional methods are total particulate matter (TPM) or solvent soluble/organic particulate matter (e.g., benzene-soluble matter (BSM)). Recently more often the amount of total organic matter (TOM) in both aerosol and vapour is determined. No methods exist that specifically characterise the vapours and aerosols released from hot bitumens.
Internal exposure to polycyclic hydrocarbons from bitumens is frequently monitored by determination of urinary 1-hydroxypyrene. However, this biomarker is not specific for bitumen exposure and may easily be confounded by other sources of polycyclic aromatic hydrocarbons in the working place.
Up to the end of 2006, in the Netherlands, there was an administrative occupational exposure limit for asphalt smoke (bituminous) of 5 mg/m3 (8-h TWA). In the United Kingdom, occupational exposure standards were established of 5 mg TPM/m3 (8-h TWA) and 10 mg TPM/m3 (15-min STEL). The National Institute for Occupational Safety and Health (NIOSH, the USA) has recommended an exposure limit of 5 mg TPM/m3 (ceiling, 15-min TWA). The American Conference of Governmental Industrial Hygiene (ACGIH) recommended a Threshold Limit Value of 0.5 mg BSM/m3 of the inhalable aerosol.
In Germany, bitumen (vapour and aerosol) is classified as a category 2 carcinogen. Both ACGIH and NIOSH indicated that there is a cause of concern that bitumen fumes may be carcinogenic (Class A4, ACGIH; ‘Ca’ notation, NIOSH).
Germany has assigned for bitumen (vapour and aerosol) a skin notation.
The absorption, distribution and excretion of constituents present in bitumen vapours and aerosols depend on their properties and mutual interactions. Constituents such as polycyclic aromatic hydrocarbons and aliphatic hydrocarbons may be absorbed through the epithelia of the respiratory tract and are distributed throughout all internal organs, particularly in those with high fat contents. PAH are metabolised, whereas long-chain aliphatic hydrocarbons are not expected to undergo extensive metabolism. PAH and aliphatic hydrocarbons are released from the body in the urine and faeces; the release of aliphatic hydrocarbons is slow.
Several human, animal and in vitro studies showed that certain PAH, which were present in condensates of bitumen fume, were taken up by the skin.
Effects in humans
Prolonged skin contact with bitumen may cause skin irritation and dermatitis. Furthermore, direct skin contact with heated bitumen may cause skin burns.
The main health complaints after acute and repeated bitumen fume exposure under road pavers and roofers are respiratory tract effects, such as nose and throat irritation, coughing, dry throat, nasal discharge, nose bleeding, shortness of breath, and asthma. These effects appear to be of mild severity and transient in nature. However, the interpretation of these findings proved not to be easy. A main problem is namely that in practice workers, who use bitumen products, are at the same time also exposed to other substances, or have a past exposure to coal tar pitch fumes. These substances may also produce respiratory effects, making it difficult to ascribe effects to bitumen fume exposure only. Another problem is that in most epidemiological studies health assessment included symptomatology only. For the right interpretation of the observed effects additional information, for instance on physiology and immunology, is needed.
A few attempts have been made to associate the level of bitumen fume exposure to respiratory effects, but in none of these studies a clear exposure-response relationship could be established. However, it should be kept in mind that among road paving workers respiratory health symptoms were more frequently reported at concentrations (geometric mean) at or below 1.0 mg TPM/m3 and 0.4 mg BSM/m3 compared to unexposed maintenance workers (irrespective the causal agent and other possible interpretation errors).
Data on effects other than those on the respiratory tract are very limited and, therefore, should be interpreted with caution. These include symptoms of nausea, headaches, stomach pain and fatigue. Human data on neurological effects and effects on reproduction and development have not been reported.
The carcinogenicity of bitumen fume from various sources and processes has been investigated in several studies. However, the results on lung cancer risk in epidemiological studies are inconsistent. This could partly be ascribed to (confounding) factors that were present in part of these studies, such as: co-exposure to other substances; differences in use of various bitumen sources; and, absence of information on past exposure, for instance to coal tar and asbestos. In a recently well-performed large and powerful multicenter cohort study, assembled by IARC, a small but statistically significant association was found between lung cancer mortality and average bitumen fume exposure among bitumen workers. Further research is now going on to control for possible confounders in this multicenter cohort study. Overall, the evidence that bitumen fume exposure may cause lung cancer is weak. It is still unclear whether bitumen fumes or other substances, that were also present in the workplace, were responsible for the observed effects. Also evidence for an association between bitumen fume exposure and cancer at other sites than the lung is weak and inconsistent. For the committee, the weak association is a cause for concern that bitumen (vapour and aerosol) may be carcinogenic to humans.
Effects in animals
Interpretation of animal data for assessing human risk is hampered by several factors. One is that investigators used laboratory-generated bitumen fumes or condensates that did not always represent the field-generated fumes. Another is that a variety of bitumen sources, exposure routes and end-points have been used. Furthermore, only a minority of the animal studies reported on the exact exposure concentration.
A substantial number of animal studies are available on dermal exposure with laboratory derived fume condensates or solutions of bitumen in solvent. These studies revealed local skin effects, such as dermatitis after short- and long-term exposure, and ulcers and small abscesses after long-term exposure.
Concerning acute and sub-acute exposure, inhalation of paving bitumen fumes of 15 or approximately 70 mg TPM/m3 did not cause acute pulmonary inflammation in rats. In one study, nasal irritation was observed at approximately 16 mg/m3 (bitumen fume, not specified) after five days of exposure.
Rats inhaling laboratory-generated fumes of bitumen condensates for 14 weeks (6 hours/day, 5 days/week) did show irritation in the nasal and paranasal cavities at 100 mg/m3 (total hydrocarbon (aerosol and gas phase)), but no such signs of irritation were found in groups exposed to 4 or 20 mg/m3. In mice and guinea pigs exposed to bitumen fume for a prolonged time symptoms of pneumonitis, bronchitis, emphysema and bronchiectasis were reported, but no actual bitumen fume concentrations in the air were determined.
So far no evidence was found in animals supporting the hypothesis that chronic inhalation of bitumen fume induces lung tumours. However, when condensates of bitumen fume were applied to the skin, papillomas and epidermal carcinomas were found in some studies, but not in all.
Both bitumen products and condensates of bitumen fume have been tested for mutagenicity and genotoxicity in vivo and in vitro. The results of these tests were inconsistent and difficult to interpret due to the many variations in study design and exposure conditions, like the temperature at which the fumes were generated.
However, because during heating of bitumen products fumes are released that may contain carcinogenic PAH and/or PAC, it is expected that the fumes have some level of genotoxic potential, although the amount of these carcinogens in those fumes may be low.
The evaluation of adverse health effects associated with occupational bitumen (vapour and aerosol) exposure is complicated by the presence of confounding factors and other limitations. First, bitumen fume and its condensate may vary from one source to the other and may also differ amongst various applications. Second, at the moment it is still uncertain how well available exposure parameters correlate with the observed adverse health effects, and thus what can be measured to monitor workplace exposure. Third, most workers using bitumen are also exposed to other agents (e.g., coal tar (mainly in the past), silica dust, diesel exhaust). These may cause similar adverse health effects as bitumen fume and thus it is reasonable to believe that they can interfere with the adverse health effects observed upon bitumen fume exposure. To what extent these agents affect the outcome of the epidemiological studies on bitumen fume exposure is not clarified yet. Fourth, data from the literature suggest that exposure to bitumen fumes may lead to lung cancer. But lung cancer is also strongly associated with tobacco smoking. This confounding factor should be taken into account when interpreting epidemiological data, which was not always done.
Data from animal experiments are too limited to conclude on an exposure-response relationship and critical adverse health effects.
More research is needed to shed more light on the confounding and limiting factors so that more understanding in the adverse health effects of bitumen exposure can be obtained. In relation to this, the committee is aware of ongoing well-designed research in humans and animals on these subjects, and that IARC has put bitumen on a list to be evaluated within a few years. The committee expects that the newly generated data will be a valuable contribution to the risk assessment of bitumen.
Taking all the available carcinogenic and genotoxic data into account, including the expectation that fumes of heated bitumen may contain carcinogenic PAH and or PAC, the committee is of the opinion that there is a cause for concern that bitumen (vapour and aerosol) is carcinogenic. However, the evidence for carcinogenicity is weak, and further experiments are necessary before a final conclusion can be drawn. Therefore, the committee recommends classifying bitumen (vapour and aerosol) as a suspected human carcinogen that has been insufficiently investigated. This recommendation corresponds to EU classification in category 3. The situation is furthermore comparable with subcategory b of this category.
At the moment the committee considers the reliability of the available data too limited to recommend a health-based occupational exposure limit. Therefore, it abstains from making such a recommendation.
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