No chemical substance can cause adverse effects without first entering the body or coming to contact with it. There are four main ways, that is routes of exposure, for chemical substances to enter the human body:

- Inhalation (breathing in)

- Absorption (through the skin or eyes)

- Ingestion (eating, swallowing)

- Transfer across the placenta of a pregnant woman to the unborn baby

Most chemicals used at the place of work may be dispersed into the air to form dust, mist, fumes, gas or vapour and can then be inhaled. In this way also workers who are not actually handling them but stay within the reach can be exposed to a mixture of chemicals from various sources.

The key pathways of human exposure in the case of chemical accident are air or water dispersion of hazardous materials.

Air monitoring/liquid sampling is one of the most important activities in the response to chemical incidents and will play an increased role in the response to terrorist incidents. A response team that is not skilled in air monitoring is at an extreme disadvantage and, in most cases, will have to assume that a chemical hazard exists, even if it doesn't.

The use of air monitoring and sampling equipment is complicated and requires practice, and when dealing with hazardous materials there will be extreme pressure for quick results.

It is important to note that developments in the detection of hazardous materials are occurring every day, and much of this technology looks promising for use in standard HM response.