Acceptable level of CO in air is _________ ppm.

 

Carbon monoxide, $CO$ is a highly poisonous, toxic colorless gas. It is produced when incomplete combustion of wood, coal, natural gases, fuel, etc. The more the concentration of CO, the more adverse its effects are. 

Low level, 0 ppm $CO$ is a desirable range. 0-9pmm is acceptable. But above that arises the risk. 

Therefore, in the earth’s atmosphere, the acceptable level of $CO$ in the air is 9ppm.