Some will argue there are 3 routes of entry defined in industrial hygiene and others will argue there are 4. The reason to know these routes of entry is simple. It is to reduce the risk of exposing employees, students and the community to harmful substances.
The 4 Routes of Entry are:
Absorption – soaking into the skin, eyes, etc.
Inhalation – breathing into the lungs
Ingestion – swallowing the substance
Injection – piercing the skin