Allergies, which are an immune system reaction to usually harmless substances.

If this happens, the formaldehyde will immediately be transformed into a harmless substance.

A harmless substance that will not reflect polarized light.

Allergies, which are an overreaction of the immune system to usually harmless substances.

Being an element, mercury cannot be broken down or degraded into harmless substances.

"A harmless natural substance that cannot be absorbed by the human body."

"We know that some perfectly harmless substances can induce an immune response."

Instead, I think that we should consider a positive list of harmless substances and diluents.

But under these conditions, even harmless substances produce such damage.

Allergies are the result of the immune system's mistaken response to a harmless substance.