Mail Sanitization

Mail Sanitization


Mail sanitization is the process in which mail is decontaminated by exposure to radiation, high pressure, or gases. Microorganisms, such as the bacterium that causes anthrax, cannot survive these conditions. The process of mail sanitization can be applied as a precautionary measure to kill microorganisms that may be contained in the mail or to sterilize mail that is known to be contaminated.

Shortly after the September 11, 2001, terrorist attacks, the United States Postal Service (USPS) was the vehicle for bioterrorism attacks on the American people. Mail containing the anthrax bacterium was detected. Five persons who were infected by the anthrax bacterium died from the disease. As a direct result of this, the USPS developed an Emergency Preparedness Plan with the goal of protecting USPS employees and customers from future bioterrorism attacks. The plan is composed of six initiatives:

  • Prevention—reducing the risk that the mail could be used as a vehicle for bioterrorism
  • Protection and health-risk reduction—reducing the risk that USPS employees and customers could be exposed to biological weapons and preventing contaminated mail from contaminating other mail
  • Detection and identification—detection and identification of biological weapons as early in the mail stream as possible
  • Intervention—routine decontamination of mail as a precautionary measure
  • Decontamination—elimination of known biological weapons in the mail
  • Investigation—enhancement of criminal investigation methods

Mail sanitization applies to the intervention and decontamination initiatives. Achieving mail safety is no small undertaking when one considers the complexity of the USPS system and volume of mail that is processed. The postal service handles nearly 680 million pieces of mail each day. This mail is primarily letters, "flats" such as catalogs and magazines, and packages. Mail enters the USPS system in many different ways including street collection boxes, post offices, personal mailboxes, and business mail entry units. The USPS has about 300 processing and distribution centers that manage outgoing mail. The computer-controlled sorting equipment and data processing systems located at these centers distribute mail to its destination. Mail is moved from processing and distribution centers to final destination processing centers by ground, rail, or air transportation. Once at a final destination processing center, mail is then sorted and distributed to the recipients.

Methods to Sanitize Mail

The USPS is studying several different methods of decontamination to find one (or more) that can effectively sanitize mail. To be useful in mail sanitization, the decontamination method must thoroughly penetrate letters, flats, and packages but not damage the mail in any way. As of late 2002, irradiation was the only acceptable method for decontaminating mail. The addition of a sanitization step to the USPS mail system may slow down the mail delivery rate.

Ionizing radiation. Ionizing radiation kills bacteria. The energy from ionizing radiation is absorbed by molecules, breaking chemical bonds and thus destroying chemical structures. Reactive chemicals (ions and free radicals) that are produced by this process cause even further damage. This results in significant damage to the DNA and proteins of bacteria and causes them to die.

The USPS is considering three sources of ionizing radiation as candidates for mail sanitization: x rays, gamma rays, and electron beams. All three are used to sterilize medical equipment and to kill microorganisms in food to prevent spoilage. They each can kill the anthrax bacteria. Radiation can easily penetrate and sanitize most types of mail, however, it may damage film, electronics, and live objects such as seeds.

X rays are a type of high-energy electromagnetic radiation. X-ray particles, or photons, are generated when electron-dense materials are bombarded by high-energy electrons. X rays have a high-energy content and can penetrate most objects.

Gamma rays are another type of high-energy electro-magnetic radiation. Gamma rays are released by decaying radioactive compounds such as cesium 137 or cobalt 60.

An electron beam, or e-beam, is a stream of electrons that is propelled by an high accelerating voltage. The energy content of the e-beam is determined by the accelerating voltage and is lower than both x rays and gamma rays.

Of the three ionizing radiation sources, e-beam technology is the safest and most readily adaptable system for mail sanitization. In 2001, the USPS bought eight e-beam machines and planned to install them in Washington D.C. and the New York and New Jersey area. The e-beam machine requires high power and chilled water and must be contained by a structure with 10 to 15 foot-thick concrete walls and a six foot-thick concrete ceiling. As of late 2002, e-beam technology has been used to sanitize incoming federal government mail only.

Non-ionizing radiation. Types of non-ionizing radiation that have been used for sterilization are ultraviolet (UV) light and microwaves. Both are effective at killing microorganisms, but in different ways.

UV light radiation damages DNA by causing DNA strand breaks and binding DNA bases together (thymine dimers). Bacteria with damaged DNA cannot reproduce or survive. UV light radiation cannot penetrate objects and is used to sterilize surfaces and air only. In addition, some microorganisms are resistant to the effects of UV radiation. Therefore, UV radiation is an unacceptable method to sanitize mail.

Microwave radiation is a low energy non-ionizing radiation. The energy in microwaves is transferred to water molecules in microorganisms. The water molecules heat up and the heat is transferred to surrounding mole-cules, thereby damaging and ultimately killing the micro-organism. Microwave radiation sanitization has shortcomings. Most importantly, it is difficult to control the heating effects and it is common to have "hot spots" and "cold spots." Also, the water content of dormant bacterial cells (spores) is low, so microwave radiation may not destroy them. Microwave radiation would be ineffective for mail sanitization.

Ultra-high-pressure sterilization. Ultra-high-pressure (UHP) sterilization is accomplished by applying a pressure of almost 100,000 psi, which causes physical changes to DNA and proteins. The resulting cellular damage kills the microorganisms. Without added heat, UHP sterilization techniques may be less effective against bacterial spores than against growing bacterial cells.

UHP sterilization is being developed for the food industry and has been shown to be effective on both solid and liquid foods. The UHP sterilization cycle time can be less than 30 minutes and the process is non-destructive to the object being sterilized. This method could be applied to mail as a sanitization method, however, a UHP sterilization system for mail will not be available for several years.

Gaseous treatment. Certain gases have anti-microbial properties and are used for disinfection and sterilization. The USPS has identified chlorine dioxide, ethylene oxide, methyl bromide, and ozone as candidates for gaseous sanitization.

  • Chlorine dioxide: an oxidizer that disrupts proteins and protein synthesis. It was used to disinfect an office building that was contaminated with anthrax spores.
  • Ethylene oxide: an alkylating agent that damages proteins, leading to bacterial or viral death. It is used to sterilize medical equipment.
  • Methyl bromide: a toxic pesticide that has been used to fumigate large buildings. It is an ozone-depleting chemical and will not be used after 2006.
  • Ozone: an oxidizing agent used to disinfect water and decontaminate unoccupied spaces. Its effect on spores is variable depending upon the specific bacterial strain.

Large amounts of gas would be needed to sterilize mail and it is not evident that gases can kill microorganisms within sealed letters, flats, and packages. Gaseous sterilization of mail is not currently a viable option for mail sanitization.



"Months After Anthrax Scare, Mail-Safety Goals are Unmet." USA Today. (August 29, 2002): 12a.

"USPS Builds to Sterilize Mail." Engineering News-Record no. 247 (November 26, 2001): 11.


United States Postal Service. < > (December 14, 2002).


Anthrax, Terrorist Use as a Biological Weapon
Bioterrorism, Protective Measures
Decontamination Methods
Postal Security
Postal Service (USPS), United States
Radiation, Biological Damage
September 11 Terrorist Attacks on the United States

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