It seems that PROPERLY secured MODERN ie post- 1980s tires (partially filled with cement and with a hole drilled in the top to allow the trapped air to escape) are biologically "inert" at least I fresh water as far as have read from the few scientifically DOCUMENTED reports that have read so far.... The jury is still "out" as far as them "leaching" chemicals using them in salt water. PRE- 1980s tires apparently can "leach" because they were made without those chemicals that allow longer wear and exposure. If the tire has any sign of "crumbling"/ deterioration it is almost certainly a pre-1980s tire and would NOT recommend using it...