Dry heat sterilization technique requires a longer exposure time (1.5 to 3 hours) and higher temperatures than moist heat sterilization. Various available methods of dry heat sterilization are; hot air oven, incineration, flaming (wire loop), etc.
Dry heat ovens are used to sterilize items that might be damaged by moist heat or that are impenetrable to moist heat (e.g., powders, petroleum products, sharp instruments).
Sterilization is defined as killing or removal of all microorganisms, including bacterial spores.
Principle of Dry heat sterilization using HOT AIR OVEN
Sterilizing by dry heat is accomplished by conduction. The heat is absorbed by the outside surface of the item, then passes towards the center of the item, layer by layer. The entire item will eventually reach the temperature required for sterilization to take place.
Dry heat does most of the damage by oxidizing molecules. The essential cell constituents are destroyed and the organism dies. The temperature is maintained for almost an hour to kill the most difficult of the resistant spores.
The most common time-temperature relationships for sterilization with hot air sterilizers are
- 170°C (340°F) for 30 minutes,
- 160°C (320°F) for 60 minutes, and
- 150°C (300°F) for 150 minutes or longer depending on the volume.
Bacillus atrophaeus spores should be used to monitor the sterilization process for dry heat because they are more resistant to dry heat than the spores of Geobacillus stearothermophilus. The primary lethal process is considered to be the oxidation of cell constituents.
There are two types of dry-heat sterilizers
- the static-air type and
- the forced-air type.
The static-air type is referred to as the oven-type sterilizer as heating coils in the bottom of the unit cause the hot air to rise inside the chamber via gravity convection. This type of dry-heat sterilizer is much slower in heating, requires a longer time to reach sterilizing temperature, and is less uniform in temperature control throughout the chamber than is the forced-air type.
The forced-air or mechanical convection sterilizer is equipped with a motor-driven blower that circulates heated air throughout the chamber at a high velocity, permitting a more rapid transfer of energy from the air to the instruments.
Advantages of dry heat sterilization
- A dry heat cabinet is easy to install and has relatively low operating costs;
- It penetrates materials
- It is nontoxic and does not harm the environment;
- And it is noncorrosive for metal and sharp instruments.
Disadvantages for dry heat sterilization
- Time-consuming method because of a slow rate of heat penetration and microbial killing.
- High temperatures are not suitable for most materials e.g. plastic and rubber items cannot be dry-heat sterilized because temperatures used (160–170°C) are too high for these materials.
- The time and temperature required will vary for different substances and overexposure may ruin some substances.
16 thoughts on “Dry-Heat Sterilization: Principle, Advantages, Disadvantages”
I am a member of a mission group that runs dental clinics in third world countries. The question I am asking is being debated. Does the CDC recommend or prohibit household type ovens ether convection or standard for the sterilization of dental tools?
The CDC recommendations for infection control in dental healthcare settings (1) states; “use only FDA cleared medical devices for sterilization and follow the manufacturer’s instructions for correct use”. Household oven are not cleared by the FDA.
1. CDC. Guidelines for infection control in dental healthcare settings – 2003. http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5217a1.htm (Recommendation VI. A. 1).
Thanks for this article. I recommended this article to our clients who are little confused in buying a gravity convection oven or a forced air sterilizing oven.
What would be the lowest temperature for dry heat sterilisation, if the time was not limiting parameter.?
You cite 170, 160 and 150°C condition, but would 140, 130 or 120 allow for proper sterilisation if given enough time (hours, days …weeks ?).
Can you please tell me as which 2 Wraps that are unacceptable for use in Dry Heat Sterilization and why they are
How does Gamma Radiation deactivate micro-organisms?
Ranie Osman (firstname.lastname@example.org)
Dear Ranie Osman
Thank you for your question. Let me address your question in broadly. The ideal wrapper (or packaging material) must allow penetration of the sterilant (heat in this case), provide protection against contact contamination during handling (i.e. puncture resistance), provide an effective barrier to microbial penetration etc. This will ensure sterility of the processed item after sterilization. Thin paper bags which may tear or materials which may charred or materials that can not withstand sterilization temperature must not used as wrapper. Gamma rays (ionizing radiation) kill microorganisms by damaging their DNA.
Thanks for the information.
One would ordinarily reason that the longer the heat sterilizer is run the higher the temperature generated. Your posting of reaching 170°c in 30mins and 130°C in 150 minutes or there about is in the co ntrary. Can you please explain or review this position.
Dear Daniel, this is the holding temperature, i.e. your articles should be subjected to this temperature for that duration to get sterilized.
Thanks for the article.
What can we use to sterilise Multani Mitti to make face pack (we use water and glycerine for making paste).
sometimes, the Face pack was catching fungus. So I heated the dry clay to 120oC for 1 hour in static air type oven –
I noticed following :-
Multani mitti was smoking a little and bottom side clay became dry brown as compared to middle or top side.
The absorbtion of water and glycerine became less (was leaving more water compared to before, after face pack was mixed).
What temperature / time do you suggest for multani mitti and other face clays? Or any other alternate procedure to sterilise the face pack clays?
Your help will be much appreciated.
Can we use Dry heat to sterilize Tarson Plastic filtration assembly?
can we use this technique to sterilize nutrient agar media
Hot air oven is not suitable for sterilization of culture media due to its low penetration power. If you do not have autoclave in your settings, you can either use pressure cooker. Another method applicable can be intermittent heating (Tyndallization) in water bath, which is generally tedious approach.
can dry heat sterilizer be used to sterilize manicure and pedicure tools? can I be sure that dry heat will kill the spores of fungi, HIV and hepatitis? what kind of regime should i use (time, temperature )? thank you .
what kind of wrap/container can be used when very fine scissors to sterile by dry heat?
Can I use convection oven 70 degree F for 60 min. to sterilize N95 mask?
What are your thoughts on UV-C light? Can it be used to sanitize N-95 Masks or any type of filtration masks? Thank you