In a very well written and interesting study Saliet al found an increased mortality due to ischaemic heart disease (IHD) among European workers producing rock or slag wool and continuous filament after 30 years since first employment.1 These types of fibres belong to a group called man made vitreous fibres (MMVF). Other types in this group are ceramic fibres and glass fibres.

These groups of European workers have been compared with the national death rates of the respective countries.1 This comparison is most often regarded as an underestimation of the true risk as the general population includes sick and disabled people unable to work. This underestimation is well known as the healthy worker effect.2

During the past decade fibrinogen has emerged as an important risk factor for IHD.3 Fibrinogen is a general indicator of inflammation in the human body.

European workers from the United Kingdom, France, and Germany exposed to ceramic fibres were investigated for respiratory function.4 Workers who currently or formerly smoked had a significant decrease of forced expiratory volume in 1 second (FEV₁) which was related to cumulative exposure. There was also a decrease of vital capacity (VC), which was not significant. A relation between decreased lung function (VC and FEV₁) and increased concentrations of fibrinogen has been found in a study of 788 Swedish men.5 Several studies have found a relation between a decreased lung function expressed as VC6 or FEV₁ 7 and IHD.

Among the European workers exposed to ceramic fibres there was a relation between increasing exposure to respirable fibres and two degrees of breathlessness from the MRC respiratory questionnaire.4 Some studies have found significant associations between breathlessness and cardiovascular mortality.8 9

An increased prevalence of chronic bronchitis has been found among United States workers exposed to fibreglass after adjustment for exposure to asbestos and smoking (relative risk (RR) 2.3, 95% confidence interval (95% CI) 1.1 to 4.9).10 A Finnish study has shown that people with chronic bronchitis have an increased risk of coronary disease and coronary deaths.11

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 concentrations of fibrinogen will increase the likelihood for blood clotting and thereby the risk for myocardial infarction and IHD.12 13 Possible indicators of some inflammatory process in the lungs could be decreased lung function, breathlessness, and chronic bronchitis. This hypothesis will be supported if workers exposed to MMVF have a higher concentration of plasma fibrinogen than non-exposed workers with control for other possible confounders such as smoking habits.14 Sali et alsuggested further investigations on the relation between an increased mortality from IHD and exposure to rock or slag wool and ceramic fibres1 and this is one way of performing such further studies.