I’d never heard the term “micromort” before. It’s a probability: a one-in-a-million probability of death. For example, one-micromort activities are “travelling 230 miles (370 km) by car (accident),” and “living 2 days in New York or Boston (air pollution).”
I don’t know if that data is accurate; it’s from the Wikipedia entry. In any case, I think it’s a useful term.
EDITED TO ADD (2/12): Discussion here.