Faced with imminent new global marine pollution rules, shipping companies and insurers are puzzling over the risks.

To reduce emissions of toxic sulphur that cause premature deaths, shipowners who have long relied on the dirtiest residues of oil extraction will have to either switch to low-sulphur fuel or install exhaust gas cleaning systems from Jan. 1.

Neither option has been fully tested for long, and some problems have already been reported, both with the more expensive new fuels and with devices known as scrubbers which extract the sulphur on board.

Interviews with key players in the industry show varying levels of alarm at potential risks, which they say range from…