DISINFECTION 237
sporicidal. Moreover, alcohols may reduce the sporicidal
activity of aldehydes.GASEOUS ANTIMICROBIAL AGENTSGaseous agents may play an important role in sterilization of
certain types of medical equipment and of components used
in outer space research. However, only two gases (ethylene
oxide and formaldehyde) are used extensively for the ster-
ilization of medical products. Other gases (methyl bromide,
b -propiolactone and propylene oxide) are not used as routine
methods. Appropriate measures must be taken to counteract
toxicity to humans (Christensen and Kristensen, 1982).Ethylene OxideEthylene oxide (EO) exists as a gas which is soluble in rubber,
water, oils and several organic solvents. Chemically, it isCH 2 CH 2.
OIts inflammability when in contact with air is overcome by
using mixtures of EO with carbon dioxide or with fluoro-
carbon compounds (Phillips and Kaye, 1949; Barwell and
Freeman, 1959; Freeman and Barwell, 1960). EO diffuses
freely through paper, cellophane, cardboard and some plas-
tics, but less rapidly through polythene; it cannot penetrate
crystalline materials (Opfell and Miller, 1965). The antimi-
crobial activity of EO has been dealt with in many papers and
reviews (Phillips, 1949, 1958, 1961, 1977; Phillips and Kaye,
1949; Kaye, 1949, 1950; Kaye and Phillips, 1949; Kaye
et al., 1952; Phillips and Warshowsky, 1958; Thomas, 1960;
Bruch, 1961; Sutaria and Williams, 1961; Russell 1971; 1976;
Kelsey, 1961, 1967; Sykes, 1970; Hoffman, 1971; Kereluk,
1971; Ernst, 1974, 1975; Hugo and Russell, 1982).
Several factors are known to influence the antimicrobial
activity of EO (Christensen and Kristensen, 1982):1) Concentration. As would be expected, the higher
the concentration of EO (expressed as mg/l,which refers to the actual amount present in the
sterilizer) the more rapid is the rate at which
microorganisms are killed. However, even at
high concentrations, EO is only slowly lethal,
and long periods may, therefore, be needed for
sterilization to be achieved.
2) Temperature. The lethal activity of EO increases
with a rise in temperature. It has a temperature coef-
ficient of 2.74 for each 10°C rise in temperature.
3) Type of organism. EO gas will kill bacteria and
their spores, yeasts, moulds and their spores,
and viruses (Griffith and Hall, 1938), and resis-
tant strains have not been developed. In contrast
to many other chemical substances which are
considerably less effective against spores than
against vegetative cells, bacterial spores are only
about 2–10 times as resistant as the latter to EO
(Toth, 1959; Phillips, 1958) and in some cases,
e.g., B. stearothermophilus (Thomas et al., 1969)
even less resistant. These results imply that EO
can freely penetrate the outer layer(s) of the
bacterial spore, although experimental results in
support of this contention are sadly lacking.
In addition to its antibacterial and antifungal
activity, ethylene oxide is also effective against
rickettsiae and viruses (Hoffman, 1971).
4) Relative humidity (RH). Of all the factors which
influence the activity of EO, RH is probably the
most important. The optimum RH is considered
to be c. 28–33% (Schley et al., 1960), and EO
is considerably less microbicidal at high RH
and in relatively dry air. The correct RH may
be achieved to prehumification of the steriliza-
tion chamber (Halowell et al. 1958; Ernst and
Schull, 1962). However, the moisture content of
microorganisms themselves, as well as the RH
of the environment, is also important. Bacterial
cells which have been desiccated and then equil-
ibrated to successively high RH values contain
less water and are more resistant to EO than cells
which have not been desiccated but have instead
been allowed to dry naturally until equilibrated to
the same RH values. To overcome this resistanceOO OOOOO ONaphthaquinone9,10-Anthraquinone9,10-PhenanthrenequinoneBenzoquinoneFIGURE 4C004_002_r03.indd 237C004_002_r03.indd 237 11/18/2005 10:19:30 AM11/18/2005 10:19:30 AM