Contact between a contaminant and an organism can occur through any route. The possible routes of exposure are: inhalation, if the contaminant is present in the air; ingestion, through food, drinking or hand-to-mouth behavior; and dermal absorption, if the contaminant can be absorbed through the skin.
Exposure to a contaminant can and does occur through multiple routes, simultaneously or at different times. In many cases the main route of exposure is not obvious and needs to be investigated carefully. For example, exposure to byproducts of water chlorination can obviously occur by drinking, but also through the skin, while swimming or washing, and even through inhalation from droplets aerosolized during a shower. The relative proportion of exposure from these different routes cannot be determined a priori. Therefore, the equation in the previous section is correct in a strict mathematical sense, but it is a gross oversimplification of actual exposures, which are the sum of the integrals of all activities in all microenvironments. For example, the equation would have to be calculated with the specific concentration of a compound in the air in the room during the time interval. Similarly, the concentration in the ambient air would apply to the time that the person spends outdoors, whereas the concentration in the food that the person ingests would be added. The concentration integrals via all routes would be added for the exposure duration, e.g. hourly, daily or annually as
where  is the initial time and  the ending time of last in the series of time periods spent in each microenvironment over the exposure duration.