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Paolo Boffetta and his coworkers presented a comprehensive cohort study comprising 6784 male and 265 female workers from four mercury mines and mills in Spain, Slovenia, Italy, and the Ukraine.1 The expected number of deaths were derived from the national rates specific for sex, age, and calender period. Slovenia was the only country with an increased mortality of ischaemic heart disease among men (SMR 1.66, 95% CI 1.35 to 2.02). In the Slovenian mine, dust measurements showed concentrations between 30 and 70 mg/m3 with 10–35% free silica in the 1960s, and about 40 mg/m3 in the 1970s. An increased mortality from pneumoconiosis was present in all countries. Mortality from ischaemic heart disease was positively correlated with duration of employment but not with cumulative exposure to mercury. Smoking habits was an unlikely confounder as mortality from diseases strongly asssociated with tobacco smoking—such as bronchitis, emphysema, and asthma—was not increased and mortality from lung cancer showed only a small increase (SMR 1.19). The purpose of this letter is to discuss further a possible relation between silica exposure and ischaemic heart disease (IHD).
A recently published study comprised 4626 industrial sand workers exposed to crystalline silica.2 The study showed a higher standardised mortality ratio regarding IHD (SMR 1.22, 95% CI 1.09 to 1.36). Smoking might hypothetically be responsible for 2–4% of this increase.
A Swedish case–control study comprised 26 847 men with myocardial infarctions; for each case, two controls were selected from the study base through random sampling, stratified by age, county, and socioeconomic group. The second highest risk was found among stonecutters and carvers (RR 1.9, 95% CI 1.1 to 3.4). This high risk could not be explained by differences in smoking habits.3
A cohort consisted of 597 miners from North Karelia in Finland employed for at least three years in a copper mine or a zinc mine.4 The excess mortality was mainly due to IHD; 44 were observed, the expected number was 22.1 based on the general male population, and the North Karelian expected number was 31.2 (p < 0.05).
A cohort of 3971 white South African gold miners was followed from the beginning of 1970. Most of the miners worked that year and the age of the workers was 39–54 years. The participants of the study were followed for nine years. A case–referent analysis was conducted comprising the miners who had had at least 85% of their service in gold mines. Ten years of underground mining was associated with a risk ratio of 1.5 (p = 0.004) regarding IHD after adjustment for smoking, blood pressure, and body mass index.5
A large cohort comprised 68 241 miners as well as pottery workers from south central China.6 The participants were employed between 1972 and 1974 and followed until 1989. There was an increased mortality due to IHD (SMR 1.25, 95% CI 1.05 to 1.45). Smoking habit was unlikely to be responsible for this risk as the mortality from lung cancer was lower than expected (SMR 0.8, 95% CI 0.7 to 0.9). There was no significant trend regarding mortality due to IHD when medium and high dust exposed workers (RR 1.16) were compared with low dust exposed workers (RR 0.65). Silicotics did not have an increased mortality due to IHD (RR 1.1, 95% CI 0.7 to 1.8).
A general hypothesis about exposure to inhaled particles and the occurrence of IHD can be expressed in the following way. Long term inhalation of particles retained in the lungs will create a low grade inflammation associated with an increase in plasma fibrinogen. The high concentration of fibrinogen will increase the likelihood for blood clotting and thereby the risk for myocardial infarction and IHD.7,8 A high concentration of fibrinogen in plasma is an established risk factor for IHD.9 An increased concentration of fibrinogen has been observed among tunnel construction workers after a workshift with a dust exposure of less than 2 mg/m3.10 Thus dust exposure in general and silica exposure in particular could be interesting to discuss in relation to ischaemic heart disease in this study by Boffetta and coworkers.1