Assessment of health effects of long-term occupational exposure to tunnel dust in the London Underground

This study aimed to provide an informed opinion on the risks to health of workers’ long-term exposure to tunnel dust in the London Underground (LU). Dust mass concentrations and particle numbers were sampled at three stations underground and in train cabs on three lines, selected by trade union representatives. Size and composition of the dust was analysed; likely maximal exposures of staff and passengers were estimated; and toxicity was tested (in comparison with other dusts) using in vitro methods. Results were reported to representatives of LU management and unions before this report was finalised. Results showed that tunnel dust differs markedly from outdoor particles; consequently, risks from outdoor particles are misleading for estimating its health effects. Tunnel dust is coarser, being generated by interaction of brakes, wheels and rails rather than by combustion, with higher mass concentrations (130 – 480μg/m3 PM2.5) and lower particle numbers (14,000-29,000 particles/cm3). It comprises approximately 90% iron, 1-2% quartz and traces of other metals. Toxicology showed the dust to have cytotoxic and inflammatory potential at high doses, consistent with its composition largely of iron oxide. The concentrations underground are well below allowable workplace concentrations for iron oxide (we estimated maximal exposures of about 200μg/m3 over 8 hours; the occupational exposure standard for welding fume, as iron oxide, is 5mg/m3 over an 8-hour shift) and so are unlikely to represent a significant cumulative risk to health of workers or commuters. “”