Procedure for Killing Bacillus anthracis Spores in Mail

Bacillus spores, including those of Bacillus anthracis (commonly referred to as “anthrax spores”), are killed after 2 hours exposure to dry heat at l60ºC (320ºF). This is well documented in the literature and is agreed upon by Bacillus spore experts.

Most mail will withstand this temperature with little or no effects on the quality of the paper. Most plastic windows of envelopes merely turn loose and shrink to form a small plastic pellet, which falls back into the envelope. Stamps heated at 320ºF were found to remain firmly in place and envelopes tested remained closed. As for magazines with glossy finishes, there may be a slight alteration of color, but not significant in the majority of tests conducted. Slight odors may be released during the heating process from certain paper products, which can be dispelled with exhaust fans or ventilation.

Mail may be transported to and heated in conventional kitchen ovens using foil containers or possibly heat resistant bags. Turkey basting or oven bags appear to be ideal heat resistant bags and are generally available at local grocers. Mail can be transferred from the post office or mailboxes in the bags, closed with the provided twist-ties, and placed in the oven. This practice also minimizes the contamination of vehicles in those cases where mail is picked up at a post office box and then taken to a residence or business in a vehicle. Caution should be used to prevent papers from coming in contact with heating elements in electric stoves or flames in gas stoves. Do not use a broiler. Ovens with poor temperature control (toaster ovens) should not be used. An oven thermometer may be required to assure that the temperature is 320ºF in certain ovens. Microwave ovens have been shown to ignite certain papers and present problems with respect to metal staples in the mail, thereby preventing their use. With respect to conventional ovens, it is suggested that pilot experiments, with small quantities of mail, be performed to assess any risks. A fire extinguisher should be readily available during this assessment. Large bundles of mail may insulate the inner portion of the bundle and require additional heating time, however, no guidance is offered at this time. When in doubt, add 30 minutes to the baking time. The author assumes no responsibility for loss of plastic items (including credit cards), fires, odors or other damage. By placing mail in containers during the heating process, there should be no contamination remaining, the oven is safe for other uses and the oven bags can be reused.

Plastic wrappers, around magazines and similar items can be sterilized with household bleach diluted 1:10 (1 part bleach and 9 parts water). Household bleach used should contain approximately 5% sodium or calcium hypochlorite. Protective rubber or latex gloves should be used while handling contaminated mail and sterilization of plastic with bleach. Further personal protection is left to the discretion of the individual or their supervisor. Mailboxes and mail handling areas may also be wiped down with the diluted bleach periodically and then rinsed with water if necessary after a 5-minute contact time with the bleach. Small disposable bleach wipes are now commercially available in grocery stores.

It is known that Baci1lus anthracis spores are less resistant than spores of other Bacillus species to moist heat. Therefore, studies are being performed to determine if increases in relative humidity during the heating process (possibly accomplished by the addition of a small amount of water to the oven bag) can be used to lower the required temperature, reduce the time, and perhaps also reduce concerns about associated risks While there may be opportunities for fine-tuning the process, the advantages of this approach are, that the process is low-tech, immediately available, and can be performed in residences and offices. It is based on firm scientific data with respect to temperature and time required for killing Bacillus anthracis spores and with respect to initial experiments which have shown that spores from Senator Daschle’s office are killed well within the 2 hour heating period.

John W. Ezzell, Ph.D., Chief

Special Pathogens Branch

USAMRIID, Fort Detrick MD 21702-5011