Nanoparticles. They irritate your lungs in ways that are very similar to asbestos irritation. And of course, it's your body's reaction to the irritation that kills you.
Example: http://pubs.acs.org/doi/abs/10.1021/acsnano.7b01873
Example: http://pubs.acs.org/doi/abs/10.1021/acsnano.7b01873
I guess that the problem with a lot of these things is that we don't have good (enough?) predictive models for long-term damage.