There is no specific rule about how deep a wound needs to be to develop tetanus. Generally, any relatively contaminated wound that is sharp and deep should receive a prompt tetanus injection, as tetanus is a strictly anaerobic bacterium. Generally, if a wound is particularly prone to forming a locally hypoxic environment, active immunization should be pursued. Usually, the following types of wounds should be given high attention: deep puncture wounds, bullet or shrapnel injuries, open fractures, or crush injuries. Burns, frostbites that require surgical intervention, and wounds that have not been treated in a timely manner after more than six hours should also be taken seriously. Attention should be heightened if there are foreign objects in the wound or a significant amount of necrotic tissue, especially if contaminated by surrounding dust or animal feces.