The World Health Organisation has found fumes from diesel engines can cause an increased risk of cancer.
People exposed regularly to diesel exhaust fumes have about a 40% higher risk of certain cancers, according to the study.
The rate of lung and bladder cancers for miners, railway workers and truck drivers is one of the reasons why the WHO is classifying diesel exhaust fumes as a known carcinogen, in the same category as plutonium and alcohol.
Diesel exhausts were previously labelled as probably carcinogenic to humans.
The BBC reports the impact on the wider population, which is exposed to diesel fumes at much lower levels and for shorter periods of time, is unknown.
But Dr Christopher Wild, of the the WHO's International Agency for Research on Cancer, says the evidence is compelling and developing nations need to pay attention.