Ultraviolet (UV) light is frequently employed to reduce airborne microbial contamination and for surface sterilization.
RADIATION
Ultraviolet
(UV) light is frequently employed to reduce airborne microbial contamination
and for surface sterilization. This is usually achieved by utilizing mercury
vapor lamps with an emitted light of 2.537 x 10-7 m.
Radiation
sterilization includes the use of the ionizing radiation of X rays and γ rays. X rays are derived from
bombardment of a heavy metal target with electrons. γ Rays are obtained from atomic
nucleus decay from the excited state to the ground state.
The
energy evolved from radiation can be equated to photon behavior as follows:
E
= hv
And
v = C/λ
where
E and v are the energy and frequency
of a photon, respectively, h is
Planck’s constant, and C and λ
are the speed and wavelength of light, respectively.
The
energy absorbed from the radiation sources equates to the dose.
1 rad = 100 ergs/g of material absorbing
= 6.24 x 1013 eV/g
= 2.4
x 10-6 cal/g
There
are a variety of radiation sources. 60Cobalt (60Co)
decays to 59Co in the core of a nuclear reactor to emit two photons
(1.17 x 106 eV and 1.33 x 106 eV) and an electron (0.31 x
106 eV). The halftime for decay is 5.3 years. 137Cesium (137Cs)
decays emitting one photon (0.661 x 106 eV). Cesium has a 33-year
half-life. An electron beam can be accelerated to energy equivalent to 5 x 106
to 10 x 106 eV. At energies below 5 x 106 eV, penetration
is insufficient for sterilization. Energy above 10 MeV may induce untoward
effects of radioactivity. The depth of penetration of radiation can be
correlated with energy levels. For example, materials with density equivalent
to water (ρ = 106 g/m3) are
penetrated 5 x 107 m/eV. 60Co gives rise to radiation
that penetrates 0.3 m through water. Accelerating electrons have high dose
rate, and exposure is only required for seconds. 60Co has a lower
dose rate, so an exposure for hours is required.
Ionizing
radiation arises from the photoelectric effect, the Compton effect, or ion pair
production. γ Radiation causes
local and intense damage and may break chemical bonds. The primary target is
the deoxyribonucleic acid (DNA) of the microorganism. In addition, free
radicals may be formed, that is, peroxides that result in intracellular and
extracellular peroxides by a chain reaction that cause damage.
Damage
depends on the amount of energy absorbed relative to the number and resistance
of the microorganisms being irradiated. Unicellular organisms have greater
resistance than multicellular ones. Gram-positive bacteria have greater
resistance than gram-negative bacteria. Finally, bacterial spores have greater
resistance than vegetative forms. Viruses are more resistant than bacteria. The
energy required to reduce the population of viruses by 90% (D-value) is 5 x 105
rad. Fungi are equivalent to bacterial spores in their resistance.
To
evaluate the dose, a number of parameters must be known. What magnitude of
source (e.g., 60Co) is available? A typical source ranges between
500,000 and 2 million curies (Ci) where 1 Ci is 3.7 x 1010
disintegrations/sec. The product geometry and the speed of the conveyor
carrying it to the source must be known. The dose can be evaluated by a variety
of dosimetric techniques. In bulk or ampules containing liquids, ferric
ammonium sulfate and ceric sulfate can be used to show an absorbance change,
evaluated by UV spectrophotometry. This is only accurate for 60Co
and 137Cs.
Radiochromic
solids can be utilized and evaluated by visible spectrophotometry. Amber and
red polymethylmethacrylate are used to evaluate 0.1 x 106 to 1.0 x 106
or 0.5 x 106 to 5.0 x 106 rad, respectively. Nylon film
is examined for opacity following exposure and may be used to evaluate
exposures of 0.1 x 106 to 5.0 x 106 rad.
Validation
requires the determination of the bioburden and the D-value. These represent
the dose required to achieve sterilization and the estimated dose.
The
dose may be regarded as overkill if low D-values are obtained. Bacillus pumulis
exhibits inherently high resistance to γ-ionization radiation with D-values
of 0.15 x 106 to 0.22 x 106 rad. The Food and Drug
Administration would like a 12-log reduction in microorganisms. The dose
required is approximately 2.6 x 106 rad.
The
product, container, and closure must be evaluated for physical and chemical
stability. A number of radiation-induced changes can potentially occur. The
product may change in color, odor, flavor, potency, biocompatibility, and
tox-icity. The container may lose rigidity, become brittle, label adhesion, and
become leachable. The product and container may be assessed by exposure to
multiple doses and single high doses of radiation. The long-term stability can
then be evaluated under ambient storage conditions, at elevated temperatures,
and under worst-case shipping conditions.
Dose
mapping can be performed by determining the minimum radiation point in the
load. Multiple dosimeters can be used to view vertical quadrant through the
load. Dosimeters are routinely set to measure the minimum dose.
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