Toxic substances tend to produce symptoms in two time frames.
The circulatory system depends on red blood cells to transport oxygen and remove carbon dioxide from the body's tissues. Carbon monoxide bonds to the hemoglobin molecules in red blood cells, rendering them unable to transport oxygen. Although high concentrations of carbon monoxide may be acutely toxic and lead to immediate respiratory arrest or death, it is the long term physiological effects due to chronic exposure at lower levels that take the greatest toll of affected workers. This is the situation with regards to smokers, parking garage attendants, or others chronically exposed to carbon monoxide in the workplace. Even when exposure levels are too low to produce immediate symptoms, small repeated doses can reduce the oxygen carrying capacity of the blood over time to dangerously low levels.