Another low temperature method for sterilizing heat sensitive items is Formaldehyde sterilization. Formaldehyde is an organic chemical compound which is a by-product of the metabolism of many organisms and is commonly found in fresh air, rainwater, foods, industrial products and fabrics. Formaldehyde is a potentially dangerous chemical, can be toxic, allergenic, and carcinogenic!

It is considered even more dangerous than EtO and is therefore less commonly used for sterilization. It recently hit the news, after Hurricane Kartina survivors were harmed by formaldehyde inhalation in trailers provided by the Federal Emergency Management Agency (FEMA).  A recent fire at a medical waste facility caused serious health problems possibly due to Formaldehyde inhalation.  Formaldehyde is colorless but has a pungent odor. At concentrations of above 0.1 ppm formaldehyde can irritate the eyes and mucous membranes, resulting in watery eyes and at higher concentration it causes severe damage. Formaldehyde inhaled at this concentration may cause headaches, a burning sensation in the throat, and difficulty breathing. Not too pleasant, to say the least. 

Instruments that Can't stand the Heat?

Formaldehyde sterilization is used where sterilization by steam or high temperature is not possible. Typical equipment suitable for Low-Temperature Steam Formaldehyde processing are:

  • Rigid or flexible Endoscopes
  • All heat-sensitive instruments for advanced eye surgery like cryo-instruments.
  • Plastic materials: syringes, coils or tubing

Sterilization by LTSF

Formaldehyde is soluble in water and its inactivation power is greatly improved by the presence of humidity. It is most commonly used as a disinfectant, but sometimes formaldehyde is used as a sterilizing agent. The process is known as Low Temperature Steam and Formaldehyde (LTSF)

The sterilization cycle can consist of the following stages: an initial vacuum removes air from the chamber and load, followed by steam admission to the chamber with the vacuum pump running to flush out the air from the chamber and to heat the load, followed by a series of pulses of formaldehyde gas, mixed with steam. Formaldehyde gas is produced by liquid formaldehyde that is passed through a heated evaporator.  Formaldehyde is removed from the sterilizer and load by repeated alternate evacuations and flushing with steam and air. In summary, reliable sterilization using formaldehyde is achieved when performed with a high concentration of gas, at a temperature between 60oC and 80oC and with a relative humidity of 75 to 100%.

Not So Welcomed Around the World

In countries such as United Kingdom, Germany, Sweden, Denmark and Norway sterilization by LSTF is accepted, but not common. On the other hand in several countries formaldehyde as a sterilizing agent is discouraged. LTSF has not been FDA cleared for use in healthcare facilities in the USA.

Advantages of formaldehyde steam sterilization

  • Very reactive molecule, with a small difference in effectiveness between spores and cells (as is the case with ethylene oxide EtO)
  • Faster cycle time compared to EtO
  • Cost per cycle is lower than EtO
  • After sterilization most loads are available for immediate use
  • Acts as a mutagenic agent, reacting with carbonyl, thiol, and hydroxyl groups

Disadvantages of formaldehyde gas sterilization

  • The vapour is extremely irritating to the eyes
  • Weak penetrating power compared to EtO
  • Operates on a higher temperature than EtO
  • Formaldehyde residue can remain on the sterilized goods if the rinsing phase is not 100% efficient.  This can be harmful for the patients.
  • A relative humidity of ~ 75% is required in order to be effective as the gas has to dissolve in a film of moisture surrounding the bacteria
  • Not approved by FDA and only recognized in some countries