Why are active on-duty firefighters likely to experience the adverse health effects from inhaling 300 ppm of carbon monoxide faster than nonactive off-duty firefighters who inhale the same concentration of the same substance? Explain your answer using th

Why are active on-duty firefighters likely to experience the adverse health effects from inhaling 300 ppm of carbon monoxide faster than nonactive off-duty firefighters who inhale the same concentration of the same substance?  Explain your answer using the chemicals/process involved.