variation, the semivariogram values are computed for each sampling run. Since the distance between the two pairs of samplers are constant, the only variable is mean air velocity which is equivalent to the condition of air movement. The semivariogram values are analyzed using the linear regression method with mean air velocity. No significant relationship
between mean air velocity and semivariogram is found.
However, the semivariogram values were plotted with the
conditions of air movement (Figure 20). In this figure 1, 2, 3, and 4 at the X ordinate represent the conditions of air
movement; 1: the HVAC system and hoods on, 2: the HVAC system on but hoods off, 3: the HVAC system off but hoods on, and 4: the HVAC system and hoods off. The semivariogram values are highly dependent on the conditions of air movement. These results show that bacterial concentration are the most
homogeneous when the HVAC system and the hoods are on. The second most homogeneous is when the HVAC system is on but the
hoods were off. The third homogeneous is when the HVAC system
is off but the hoods are on. The least homogeneous is when the HVAC system and the hoods are off.
VI CONCLUSION
Measurement of concentration of bioaerosols in an
environment contains temporal and spatial variations. Both variations, in fact, depend on the environmental conditions,
such as air flow rate, air velocity, temperature, relative
humidity, and human activity.
Air movement in an environment may be the most important factor. High air flow rate may dilute airborne bacterial concentration in an environment. Air velocity may affect the collection efficiency of the Andersen two-stage samplers. The different air velocities in an environment may distribute bacterial concentrations inhomogeneously in the air.
In the outdoor environment airborne bacteria may be mixed very well and may eliminate spatial variation in the scale of
several meters. Air velocities in the indoor environment are
usually lower (0.02 cm/sec to 0.05 cm/sec) and may vary with location. Air velocities may depend on the physical arrangement of the indoor environment and may be enhanced by the ventilation system of the environment. In the ventilated room, spatial variation of the collected bacterial concentrations is very significant, and bacterial
In contrast to the ventilated environment, spatial variation in the nonventilated environment may increase with the distance between samplers, but the bacterial concentrations do not vary with location without human activity.
Human activity, increasing the difference of air velocities and resuspending bacteria, in the nonventilated environment, enhances spatial variation. Therefore, the collected bacterial concentrations are not equal at different locations.
Other important environmental factors are temperature and relative humidity. They are related to the collected
bacterial concentrations in an environment. The relationships
may depend on other environmental factors, such as nutrition sources, human activity, and air movement. In the indoor environment, temperature is maintained at a small range of variation (within 5°C) by the HVAC system. It is difficult to
find a good relationship among the collected bacterial
concentration, temperature, and relative humidity in the
indoor environment.
In the outdoor environment, however, the quadratic relationship may reflect that bacterial cells are damaged by oxidation of cellular components at low temperature and
temperature and relative humidity. The relationship may also respond to the difficult collection of bacteria at high and extremely low relative humidity.
Since temperature and relative humidity vary with time,
they may in part cause some temporal variation of the collected bacterial concentrations, especially the day-to-day variation. Significant temporal variation may also be contributed by the cleaning effect when sampling in a nonventilated environment. The cleaning effect is very important since the nonventilated environment, without human
activity, can be considered as a closed chamber.
In an open chamber, bacteria can freely get in and out. The cleaning effect due to sampling may not be important. The
outdoor and the ventilated environments, and the nonventilated
environment with human activity, may be considered as an open
chamber. Therefore, the collected bacterial concentrations
may vary randomly with time. Other factors may include air movement and human activity. Air velocity and human activity enhance particle transport so that the collected bacterial
concentrations vary randomly with time.
Air velocity, temperature, relative humidity, and human activity are very important environmental parameters that may affect spatial and temporal variations of the collected bacterial concentrations in an environment. Identifying these
parameters may help an investigator understand variations of sampling bioaerosols whenever air sampling is required.
VII FUTURE WORK
In this field study, spatial variations of bacterial
concentrations were found in the nonventilated and ventilated
environment. Results of the experiments cannot be completely interpreted without a controlled bioaerosol chamber study. Bioaerosol chamber studies may include; 1) determining the collection efficiency of the Andersen two-stage samplers, 2) comparing spatial variation of sampling bioaerosols with multiple microbial samplers, 3) measuring temporal variation of sampling bioaerosols, and 4) using a numerical model to simulate temporal and spatial variations of bioaerosol
concentrations.
Since bioaerosol concentrations are related to temperature and relative humidity, a bioaerosol chamber study may provide an additional explanation regarding to the relationship among the collected bioaerosol concentrations, temperature and
relative humidity. Temperature and relative humidity can be controlled in the chamber. A generalized relationship may be
observed in a bioaerosol chamber.
The effect of air velocity also needs to be studied in a wind tunnel by spraying bioaerosols and sampling. The study could be used to examine how low air velocity affects the
collection efficiency of the Andersen two-stage samplers. In fact, the relationship between low air velocity and the
collection efficiency of a sampler is not well understood for
non-viable aerosols, either.
The results in this field study combined with these bioaerosol chamber studies will contribute to designing a strategy for sampling bioaerosols in both indoor and outdoor environments.
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