The standard issue filter that is issued with the S-10 Gas Mask also known as an OEM (Original Equipment Manufacturer) filter, it is a 40mm thread fitting.
Certain filters contain activated carbon which has been impregnated with Chromium, this can present a health hazard if inhaled.
Below is information provided from Avon (the manufacturer of the S-10 and its filters)
"The activated charcoal in old S10 respirator canisters, designated L12A1, is impregnated with Chromium, which is a known carcinogen. Fortunately the charcoal is fully contained inside the plastic canister body and presents no hazard if the canister is intact. Seek the advice of local Health and Safety officials regarding disposal of old or used canisters"
The filters were designated as CBRN level of protection for use in Military or Law enforcement. It's current level of protection in comparison with other filters is up for debate.
Note on the Image with the filter with the red tape around it, indicates that the filter has been De-commissioned and not to be used for a live situation, instead for training purposes. - These are the most commonly found on sites such as eBay