The toxicity of a component depends on the amount used in the preparation. This means that certain preservatives can be very toxic in high concentrations and harmless in low quantities. The key lies in knowing how much is needed for the preservative to fulfil its function and how toxic that amount can be.
The literature shows that there are three categories of preservatives:
This category includes preservatives such as thimerosal and benzalkonium chloride (BAK), which are practically no longer found in formulas conceived to come into contact with the eye.
This group includes preservatives such as polyquaternium and polyhexamethylene biguanide (PHMB). There are biosafety tests proving their efficacy and safety, which shows that they are neither cytotoxic nor irritant. They are highly efficient in very low quantities, which is why they have been used for decades in formulas that come into contact with the eye, and the results are excellent.
This category includes chlorite and hydrogen peroxide, which are extremely efficient and completely harmless in small amounts given that they disappear as they come into contact with the eye.