Discussion

This analysis is the first to detail the characteristics of silicosis cases that have been reported to the UK national reporting scheme over a long time period. During this time, the majority of reported cases had developed silicosis from working in an industry traditionally associated with silica exposure. No clear temporal trends were seen in either the number or the age range, of the actual cases reported each year, having analysed data from over 200 workers over a 22-year period.

The limitations of using data from the SWORD reporting schemes have been discussed in detail elsewhere,13 particularly relating to the potential for underestimating disease incidence. In the case of silicosis, it is likely that this may be particularly relevant, as the majority of the disease burden would be expected to be simple silicosis that is asymptomatic. The SWORD data estimate of 30–35 new cases per year from an exposed population of over half a million workers is therefore likely to be low, although it is impossible to calculate the expected incidence without an accurate knowledge of historical exposure levels. Silicosis is predominantly a long-latency disease, which makes calculating rates of disease challenging, without knowing the true ‘at risk’ industry population size that an individual case originated from over the period of their silica exposure. A recent analysis of SWORD data attempted to allow for this by using a notional 30-year lag period between first exposure and the development of silicosis, identifying high incidence rate ratios for miners and quarrymen, glass and ceramic makers and construction workers.14 Due to the inherent limitations of this approach, our analysis did not attempt to estimate incidence, focussing simply on the numbers of cases from each industry that have been diagnosed in secondary care. A high number of cases from an industry sector in our study may therefore be due to a greater risk of disease, or a higher number of exposed workers. We found that over 90% of all silicosis cases reported from secondary care had resulted from working in one of seven industry groups that have a well-established link to silica exposure. It is not possible however to ascertain from the SWORD data where the work in these industries occurred, either within or outside the UK.

The limited data requested by the SWORD scheme for each case are a further limitation, as it does not allow disease latency to be calculated, cannot differentiate between simple or complicated pneumoconiosis and does not record whether each silicosis case reflects acute, accelerated or chronic disease. Although the conclusions that can be drawn from these data are therefore somewhat limited, it seems reasonable to assume that a younger age of diagnosis and the presence of symptoms are valid markers of disease severity.

Data available for half of the reporting period found that almost all silicosis cases reported to SWORD had a recorded symptom-onset date, suggesting complicated disease that had been referred to secondary care. However, It is possible that the true proportion of complicated disease was even higher than that found in this analysis, due to our presumption that cases without a documented symptom-onset date had simple pneumoconiosis. In some cases, however, this presumption may have been incorrect, as missing data may have simply reflected incomplete reporting.

Chronic complicated silicosis typically develops over several decades of low-dose exposure, which fits with the average age of diagnosis (61 years) seen in our analysis. Although some cases were diagnosed after normal retirement age, the majority of workers reported to have silicosis were still of working age. A number of recent international reports have highlighted the risk of silicosis in younger workers, mainly due to the use of silica containing materials in newer occupational settings. Akgun et al reported 157 former denim sandblasters with silicosis in Turkey.8 All subjects were male and had a mean age of 23 years (range 15–44). Another recent study from Turkey highlighted the importance of silica exposure as a risk factor for dental technicians’ pneumoconiosis.6 In this study, 90 cases were identified with a mean age of 35 years (SD 7). Most recently, Hoy et al reported seven male cases of artificial-stone silicosis from Australia, with a median age of 44 years (range 26–61).9 Although the mean age of diagnosis was higher for the SWORD cases over all, a significant proportion (~1 in 6) of all UK silicosis had been recognised in workers under the age of 46 years. In all cases, the younger cases had worked in an industry traditionally associated with silica exposure, rather than reflecting newer sources of silica exposure. Four cases of silicosis were reported in jewellery or dental workers (two from each occupation), but none were diagnosed below the age of 50. Although none of the ‘other’ sources of exposure or associated job titles confirmed disease due to artificial stone, it is possible that any such cases could have been reported as having occurred among ‘stonemasons’—an industry group with the highest proportion of younger workers.

Our study found that in some industry groups, particularly foundries and construction, there was an apparent increase in the number of cases with increasing age category. In contrast, in quarrying, tunnelling and the ceramic/brick industry groups this gradient was only seen in the subgroups of working-age patients, with a markedly lower number of cases reported in those over the age of 65 years. A completely different pattern was seen in miners, with two-thirds of cases being individuals of retirement age. Although it is likely that the pattern in miners relates to the decline of the UK mining industry, it is difficult to interpret the patterns seen in other industry groups, without a detailed understanding of how these industries have changed over time.

Updated silica health surveillance guidance for Great Britain was published by the Health and Safety Executive in 2016.15 This document provides clear and practical advice relating to the routine use of chest radiographs for silica-exposed workers, recommending X-rays after 15 years of exposure, and then every 3 years thereafter. It is likely therefore that an increased number of silica workers will undergo regular chest X-rays, resulting in more asymptomatic workers with simple silicosis being identified in the future. The findings of this analysis therefore represent baseline data, from which any significant demographic changes may be judged over the next decade.