Unhealthy 'ultrafine' metals are released into the subway air by train wheels.

High concentrations of microscopic metal particles were discovered in dust samples from all around the London Underground, indicating that air pollution is a widespread problem in the world's oldest subway system.

British experts admit that it is unclear whether or not these particles actually constitute a risk to human health. Experts, however, claim that this is a common occurrence in subways across the world, including in the ....