Residents of Opportunity say they want new infrastructure to prevent toxic substances from making their well water unusable.

This is done to prevent toxic substances from entering into the wider environment and affecting ecosystems.

This system acts to prevent alien substances from affecting the body in a harmful way.

The respiratory system has built-in methods to prevent harmful substances in the air from entering the lungs.

This protective layer helps prevents irritating substances from penetrating the skin and getting into its deeper layers.

And still others suggest that decreasing rates of breastfeeding have prevented important substances of the immune system from being passed on to babies.

The control banding process emphasizes the controls needed to prevent hazardous substances from causing harm to people at work.

Langerhans cells prevent foreign substances from getting into your skin.

A condom also prevents disease-causing substances from spreading from one person to another.

Thus, it serves a daily protective function preventing substances to get to the brain.