Internal combustion engines discharge a number of harmful substances, including carbon dioxide (CO2), nitrogen oxide (NOx), hydrocarbons (HC) and particulate matter. If you run a new 5 HP four-stroke outboard for one hour, you produce the same amount of NOx and HC pollution as if you were driving 38 new cars at 95 km/hr for the same length of time.
The automotive industry uses sophisticated methods to reduce pollutants, such as exhaust gas reticulation and catalytic converters. Outboards do not have any comparable systems, even new ones. This is why the level of harmful nitrogen oxide and hydrocarbon emissions is dramatically higher than cars. Though there are far fewer outboards than cars, their pollution is substantial and vastly out of proportion.
