Engine exhausts contain carbon monoxide, which is a colorless and odorless gas. It is harmful to your health in high concentrations, and even in low concentrations if inhaled for a prolonged time. 
Carbon monoxide poisoning can cause brain damage or even death.
The symptoms include nausea, headache, dizziness, ringing in the ears and loss of consciousness.
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Keep all areas well ventilated.
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Do not block exhaust outlets.
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Do not idle the engine when people are in the water or while mooring.