Ozone
Disinfection of SARS-Contaiminated Areas
Why do
we need air disinfection?
The outbreak
of SARS worldwide in March 2003 has increased people awareness of
the transmission of respiratory diseases in indoor environment.
Evidences(1) show that SARS could survive on respiratory droplets
for up to several days and people breathing air containing these
droplets will be at high risk to get the diseases. Therefore, there
is a need for a reliable and efficient air disinfection method to
decontaminate these high-risk areas.
Technologies
for air disinfection
The most common air disinfection method is using ultraviolet (UV)
radiation. UV radiation (UV-C) kills bacteria and viruses by damaging
the DNA/RNA of the cells of microorganisms. However, UV radiation
could only disinfect air close to the lamps as UV light has limited
penetration capacity. In case of SARS contaminated room, UV disinfection
alone is not adequate to provide virus-free environment for us.
Another well-known air cleaning method is to employ High Efficiency
Particulate Air (HEPA) filter. HEPA filter can capture particulate
sizes down to 0.3 microns, and so bacteria with size larger than
0.3 microns could be trapped in the filter. Although HEPA filters
are effective in reducing airborne bacteria in air, it is not effective
to remove viruses, which are nanometer (10-9 m) in size. Also, air
must pass through the filter in order for it to be cleaned. Hence
HEPA filters can only clean air that is within a short distance
of the HEPA unit. These drawbacks make HEPA filters become an unsatisfactory
candidate for disinfection of SARS contaminated areas.
Chemical disinfectants could also be used for air disinfection,
usually by means of vaporizing or spraying. However, these chemical
disinfectants are usually difficult to decompose, leaving toxic
chemical residues that are hazardous to human health.
Ozone
is a well-known powerful oxidizer which could kill microorganisms
effectively. Ozone applications in water and wastewater treatments
are well-documented and it is widely used by most of the modern
cities. Although studies for using ozone to disinfect air are
relatively limited, experimental results (2,3) indicate that ozone
could also be an effective air disinfectant as in water. For example,
Kowalski et al (2) investigated the bactericidal effects of high
ozone concentrations on E. coli and S. aureus and concluded that
more than 99.99% death rate was achieved for both species after
ozonation.
In
addition to the strong oxidizing power of ozone, properties of
ozone also help it to be an ideal aerial disinfectant. In contrast
to UV radiation and HEPA filter, ozone is a gas that could penetrate
to every corners of the room, thus it could disinfect the entire
room effectively. As ozone is unstable, it is readily converted
back to oxygen, leaving no harmful residual ozone after disinfection.
Although
ozone is success as an aerial disinfectant in laboratory experiments
(1), its effectiveness in real situation needs to be further explored.
In this article, the effectiveness of ozone in disinfection of
a conference room will be evaluated and discussed.
Disinfection
capacity of ozone
Ozone (O3)
is an unstable gas comprising three atoms of oxygen. It is unstable
because the gas will readily degrade back to its stable state, diatomic
oxygen (O2) with the formation of free oxygen atoms or free radicals.
The free oxygen atoms or radicals are highly reactive and they will
oxidize almost anything (including viruses, bacteria, organic and
inorganic compounds) in contacts, making ozone an enormously powerful
disinfectant and oxidizer.
In
fact, ozone is a much stronger oxidizer than other common disinfectants
such as chlorine and hypochlorite. The usage of chlorine or hypochlorite
in many countries has been decreased significantly due to the
possibility formation of carcinogenic by-products such as trihalomethanes
(THM) during the disinfection process. In contrast, ozone disinfection
does not produce any harmful residues, and all the residual ozone
will be converted back to oxygen within a short time. Ozone is
therefore considered as an environmentally friendly disinfectant.
Given
its superior strength and effectiveness as an oxidant and biocide,
ozone becomes one of the dominant water treatment technologies
in Europe and America. But its application in air disinfection
is not as popular as water due to the concern on ozone??s
toxicity. Ozone with concentration higher than 1 ppm has adverse
effects on human health and the use of ozone for air disinfection
is generally not recommended if people are around. Therefore,
air disinfection using ozone should be restricted to unoccupied
room only.
Procedure
for air disinfection using ozone
In order to
test for the effectiveness of ozone in reducing airborne bacteria,
a conference room with area about 12m2 was selected for testing.
As high level of ozone is required to kill viruses, bacteria and
spores, the disinfection process was carried out when humans, animals
and plants were evacuated.
Depends
on the size of the room, an ozone generator (PIE Ozonation) with
2g/hr output was chosen. The capacity of the chosen ozone generator
has the ability to maintain high concentration of ozone (0.5 ?V
5 ppm) inside. Circulation fan was placed in the room to ensure
good distribution of ozone. After closing all the windows and
doors, the ozone generator was turned on by remote device located
outside to begin ozonation process. Concentration of ozone was
monitored using a digital ozone sensor (Ecosensor). Different
levels of ozone (0.5, 2.5 and 5 ppm) were tested to determine
the optimal value for killing as much microorganisms as possible.
After turning off the ozone generator, ozone level began to drop
as it was undergoing self-decomposition to oxygen.
For
safety reason, no people should enter the room until the level
of residual ozone is below 0.02 ppm. In general, ozone concentration
drops to below 0.02 ppm in a hour after ozonation, therefore people
should wait for at least one hour (after turning off the generator)
before entering the ozonated room.
Figure
1 shows
a typical curve (concentration vs. time) during ozonation process.
As shown in the figure, the ozone concentration raise very slowly
in the initial period (the first few minutes). The delay in building
up the ozone concentration is probably due to the consumption
of ozone for oxidizing pollutants (including bacteria and viruses)
in the initial period. After oxidizing the major pollutants, ozone
concentration inside the room raise rapidly up to the desired
level. To ensure entire room disinfection, high level of ozone
was maintained for 30 minutes. When ozone generator was off, the
ozone concentration dropped gradually as ozone converting back
to oxygen.
Effectiveness
of ozone on reducing airborne bacteria
The total
airborne bacteria in the conference room was measured before and
after each ozonation. Measurement was carried out using an Andersen
N-6 single-stage sampler with Tryptone Soya Agar (Oxoid) in petri
dish. 283L of air was taken for each sampling. The petri dish was
incubated at 35oC for 48 hrs before counting. The disinfection efficiency
of ozonation at different concentration was tabulated in Table
1.
Table 1.
Reduction of Airborne Bacteria after Ozonation
| Ozone
conc. |
0.5
ppm |
2.5
ppm |
5
ppm |
Before
Ozonation
|
592
CFU/m3 |
612
CFU/m3 |
552
CFU/m3 |
| After
Ozonation |
169
CFU/m3 |
42
CFU/m3 |
57
CFU/m3 |
| Reduction
% |
71.5%
|
93.1%
|
89.6%
|
The results show that ozone is effective in reducing airborne bacteria.
At higher ozone level, the sanitizing effect increased. Over 90%
of airborne bacteria could be reduced at 2.5 ppm concentration.
Further increase of ozone concentration to 5 ppm does not beneficial
in bacteria reduction percentage.
Unlike laboratory experiments conducted by Kowalski et al (1) that
could remove 99.99% airborne bacteria after ozonation, the best
reduction percentage in our case was around 93% only. High removal
percentage could not be achieved because the conference room was
not 100% sealed. Doors should be opened briefly during each air
sampling (for placing a new agar dish on the sampler) and air exchange
from outside was unavoidable.
For safety reason, excessive high concentration ozone should be
avoided and the lowest ozone concentration that could kill most
of the microorganisms should be selected as optimum. Depends on
the contamination level, 0.5 ?V 2.5 ppm ozone level is adequate
for air disinfection.
Conclusion
Experimental
data shows that ozone is effective in reducing airborne bacteria
of unoccupied room. Over 90% of airborne bacteria could be reduced
after ozonation. As viruses are generally more susceptible to ozone
than bacteria, it could assume that all viruses are killed if large
percentage of airborne bacteria are removed. Ozone is a gas that
has good penetration capacity and powerful oxidizing power, thus
its disinfection efficiency is superior to UV radiation and HEPA
filter. As ozone disinfection is conducted in unoccupied room only
and all the residual ozone will be decomposed after the treatment,
ozone toxicity to human is therefore not a concern. Given the advantages
of strong oxidizing power, good penetration capacity and no harmful
residues left after the treatment, ozone is recommended to be used
in disinfection of SARS-contaminated environments.
References
- G?rard
V. Sunnen, SARS and Ozone Therapy: Theoretical Considerations,
http://www.triroc.com/sunnen/topics/sars.html
(2003).
- W. J. Kowalski,
W. P. Bahnfleth, and T. S. Whittam, Ozone Sci. & Eng., 20, 205-221
(1998).
- T. Masaoka;
Y. Kubota, S. Namiuchi, T. Takubo, T. Ueda, H. Shibata, H. Nakamura,
J. Yoshitake, T. Yamayoshi, H. Doi, T. Kamiki, Appl. & Environ.
Microb., 43, 509-513 (1982).
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