Exposure concentration is _________ 

The correct answer is the amount of chemicals present in the medium with which an Organism has contact.

 Key Points

• Exposure is defined as the contact of humans with a particular agent on a human outer boundary and the exposure concentration is the amount of substance making this contact.​
• Exposure concentration (C) is the arithmetic mean of the contaminant concentration throughout the exposure.
• The range of exposure concentrations is vast and the frequency of exposure may be low for some compounds such as perilla ketone and 4-ipomeanol.
• However, the exposure concentrations quantified as mass do not provide information on the number of nano-sized particles.

Therefore, the correct answer is the amount of chemicals present in the medium with which an Organism has contact.