Disinfectants are used to remove the germs and other contaminants on the surfaces of the area. They are used daily for cleaning homes, laboratories as well as food preparation areas. Hence, it is required to select the proper disinfectant according to the type of contaminants present but several physical and chemical factors also influence disinfectant procedures namely temperature, pH, relative humidity, and water hardness.
Temperature: It is one of the major factors that
influence disinfection activity. Generally, disinfectant activity increases
with the increased temperature but there are some exceptions. Generally, higher temperatures decrease surface tension and viscosity and enhance germicidal action. Furthermore, very high temperature causes the
degradation of disinfectant weakens its germicidal activity, and may produce
health problems. Example: Chlorine compounds are more corrosive in higher
temperatures and iodine tends to sublime at temperatures 49°C or above.
pH: An increase in pH improves the antimicrobial
activity of some disinfectants (e.g., glutaraldehyde, quaternary ammonium
compounds) but decreases the antimicrobial activity of others (e.g., phenols,
hypochlorites, and iodine). The pH influences the antimicrobial activity by
altering the disinfectant molecule or the cell surface.
Relative humidity: It is the most important factor
influencing the activity of gaseous disinfectants, such as ethylene oxide,
chlorine dioxide, and formaldehyde.
Water hardness: Hardness of water means a high
concentration of divalent cations present in water. Hardness reduces the rate
of killing of certain disinfectants because divalent cations (e.g., magnesium,
calcium) in the hard water interact with the disinfectant to form insoluble
precipitates.
Other factors like duration, age of disinfectants,
concentration of disinfectants, biofilms, and number of pathogens also play an
important role.
Duration: The effectiveness of a disinfectant greatly
depends on the length of time that is exposed to the area to be disinfected.
The duration of the exposure depends on targeted organisms as well as the concentration
of appropriate disinfectants. In general, longer contact times are more
effective than shorter contact times.
Age: The age of the disinfectant solution being used
also affects its effectiveness. Many chemicals have diminished potency with the
passage of time and their exposure to air and the other chemicals in the
solution.
Concentration: The chemical makeup and the potency of
the solution will affect how effective the disinfectant is at eliminating the
contaminants present. The higher the concentration of the solution,
disinfectant is more potent and therefore more effective. The disinfectant rate
by the chemical agents is depends on Chick’s law:
- dN/dt = kN
where,
dN/dt = Rate of cell destruction,
k = Rate constant,
N = Number of living cells remaining at a time “t”.
“k” depends on microorganism species, disinfectant nature,
and concentration of disinfectants.
The disinfectant’s rate is described in another equation,
Cntc = K
where,
C = Concentration of disinfectants at time t = 0,
tc = Time of contact required to kill a given
percentage of microbes; K,
n = Exponential constants.
Further, “n” depends on the nature of the disinfectants.
If n > 1, then disinfection depends on the concentration
of the disinfectant,
If n < 1, then disinfection depends on the time of
contact
“K” depends on the type of microorganisms and other
environmental factors such as pH and temperature.
Biofilms: Biofilms are thick masses of cells and
extracellular materials of microorganisms. They are protected from
disinfectants by the production of these biofilms. These biofilms are tightly
attached to surfaces of microorganisms and form resistance to disinfectants by
multiple mechanisms, including physical characteristics of older biofilms,
genotypic variation of the bacteria, microbial production of neutralizing
enzymes, pH, etc.
Number of microbes: Disinfection activity is directly
proportional to the number of microbes. The greater number of microbes present
indicates it takes more time to eliminate microorganisms. It is also further
dependent on the type of microbes. Endospores are more difficult to destroy
whereas vegetative organisms widely vary in susceptibility to different methods
of microbial control.