Carnival Corp plans to carry out further studies on open-loop exhaust gas scrubbers, weeks after claiming the devices' wash water discharge poses no harm to sea life.

The company has released a 281-sample study by classification society DNV GL saying that trace-metal levels were below emission limits for land-based industrial wastewater.

The 49-page analysis also found polycyclic aromatic hydrocarbons (PAH) -- compounds within coal and tar -- at lower levels but said more analysis is needed to ensure no harm at all.