Cigarette butts pile up in parks, beaches, streets and bus stops, places where all types of littering are frowned upon. Best estimates are that over five trillion butts are generated by smokers each year worldwide, and concern about their environmental impact has prompted studies of how they affect water and wildlife habitats. But despite their prevalence, almost no one has studied the airborne emissions coming off these tiny bits of trash.

When Dustin Poppendieck was asked to evaluate them, he was skeptical. [...]