An environmental protection agency suggests that to avoid adverse health outcomes, carboxyhaemoglobin ($COHb$) concentrations in the blood should be kept below $3.0\%$.

The agency currently recommends that individuals should not be exposed to:

• air with $>8.0{mg\; m}^{-3}$ of $CO$ for more than $8$ hours.
• air with $>25.0{mg\; m}^{-3}$ of $CO$ for more than $1$ hour.

Scientists used a computer model to calculate the $CO$ exposure levels that would lead to exactly $3.0\%$ $COHb$ in resting children and elderly individuals.

The table below shows the results of their model:

Table: CO concentration in the air / ${mg\; m}^{-3}$ resulting in $3.0\%$ COHb

The scientists concluded that the agency's exposure limits for carbon monoxide should be reduced to prevent sensitive groups from exceeding the $3.0\%$ $COHb$ threshold.