Effects of Non-Condensable Gases (NCGs) on Steam Sterilisation Processes

The dangers posed by non-condensable gases (NCGs) in steam sterilisation processes have long been underestimated. Biological indicators, or the best chemical indicators on the market, do not signal the presence of a NCG content of up to 10% in a sterilisation process so long as there is mixing of steam and NCGs in the sterilisation chamber. Already back in 1963, Bowie and Dick (1) demonstrated that steam/NCG mixtures could prove dangerous if steam resulting from condensation is being consumed on the sterile supplies and the NCGs enclosed in this steam are able to accumulate present in the form of closed packages. If the NCGs are unable to mix again with steam within closed packages containing instruments, in porous laundry packages or within lumened devices, these quantities of gas will accumulate and prevent steam penetration into these areas, thus also preventing the build-up of heat and entry of water which are prerequisites for successful sterilisation. Relatively large volumes of steam are consumed in sterilisation processes for heating (sterilisation) supplies (approx. 300 – 400 l per 10 kg materials under normal conditions). Conversely, the steam decreases its volume by a factor of 1,000 on condensing to water. This means that one litter of steam produces around one 1 ml water. The exact ratio will be determined by the pressure/temperature conditions. If a package contains 10 kg of supplies that are to be heated by means of condensation and the steam were to contain only 1% NCGs, this would result in around 4 litres NCGs accumulating in this package, and large sections of the package would not be sterilised. Therefore European Standard EN 285 regulating steam sterilisation processes for the health sector has set a very small limit for the quantity of NCGs in steam with which sterilisers should still be operated. The presence of 35 ml NCGs in 1 litre steam-condensate is given in the standard as 3.5%. But these specifications apply for a NCG-condensate mixture. In reality the mixture is present in the gas phase with a somewhat 1,000-fold greater steam volume compared with the condensate volume, hence the actual percentage in the gas phase must be around 1,000-fold less, and may reach a maximum level of around 0.003%. As borne out in practice, this is a realistic value, which if exceeded will lead to quantifiable problems.