Contamination occurs in two ways:
- Environmental contamination results from the ingress of contaminants from the surrounding production areas or even from outside environments
- Cross-contamination is defined as contamination of a starting material, intermediate product or finished product with another starting material or product during production.

Whether performing risk assessments or impact assessments there are six factors to consider in order to determine environmental impact and to inform contamination control.
- Amenability of equipment and surfaces to cleaning and sanitization
- Personnel presence and flow
- Material flow
- Proximity to open product or exposed direct product-contact material
- Interventions/operations by personnel and their complexity
- Frequency of interventions/process operations.