The number of samples will depend upon the purpose of the sampling and the type of sample.

Do you have a specific microbe to culture? Are you looking at all possible microbes?Do you know the temporal pattern? Do you want to establish the temporal pattern? 

Indoor microbes differ from outdoor. Outdoor varies with location. There are diurnal patterns and weather related patterns in addition to seasonal patterns.

Are you performing active or passive sampling? Are you performing short-term or long-term sampling? What is your analysis technique?

Thank you replying !

Well, as per the various factors associated, the motive of sampling is to conduct a survey (generate a database) for all the possible cultivable microbial load in ambient air and not for a specific one.  As such, I have to conduct a long term sampling (an year round atleast) establishing the temporal pattern including both the diurnal and seasonal variation. The mode of sampling will be impaction based active sampling.

When generating a database, The limitation is the number of samples you can analyze. The ideal is to sample at the maximum rate with a uniform temporal and spatial pattern. After sample collection for one year you would examine the results for patterns.

Uniform temporal and spatial sampling at the maximum analysis rate may not be sufficient to observe some patterns. Rapid changes may occur near sunrise or sunset or as a weather front approaches. Denser sampling may be necessary to resolve the rate of change. 

The problem with selecting a sample number or rate is knowing what resolution is necessary and knowing the variance of the microbes and the sampling process. Several options are open to you. You could perform the uniform sample collection at the maximum sampling rate and then process the total information for future studies. You could over sample and store samples for later analysis if patterns are suspected but not resolved. You could vary temporal and spatial patterns on a subset of the collection to evaluate the need to alter your sampling plan. You could maintain the uniform sampling plan with 10% of the analysis budget used for variation of the uniform plan.

The more information you have the better the plan you can make. Are there prior investigations that indicated microbe variability and density?

Thank you for valuable suggestions. Well, no previous work has been conducted for the selected sites over the region. Statistically, I need to observe the variation pattern with different meteorological parameters as well as the relationships with different variables associated. No standards or guidelines have been defined for bioaerosol sampling. So should I better conduct a preliminary sampling survey / study and observe the variations and then go with actual sampling as the problem with continuous and long term sampling (in case of bioaerosols) is generation of huge volume of sample which would be difficult to maintain for the chances of cross contamination - a hindrance for further analyses to be performed in collected samples.

You should always perform preliminary surveys. You cannot make a sample size estimation without knowing the expected variability. I suggest you perform several preliminary surveys for differing conditions.

There are guidelines for bioaerosol sampling. You can find several on the internet.

Cross contamination is always a problem. You can devise simple, disposable tools that will restrict cross contamination. Plastic film, cardboard, folded paper, toothpicks, packaging materials from chemicals and equipment, ... Test your methods. Include blanks, duplicates, and spiked samples in your testing.

Do any target organisms have time constrains for viability?

Hi Sandeep,

The following sampling strategies could be applied for the bio aerosol sampling when you focus the cultivable load,

Impaction based sampling is considered reliable for sampling.

There is an instrument called Biosampler (SKC). You can use this sampler. With the sampler that involves impaction technique, you have the sampling time constraints of 20 mins. So the air is sucked in to a saline ( would'nt kill the cells) for 20 mins. You then bring the sampled saline and plate it in a common bacterial culture media. And then do the counting and establish the CFUs/m3.

You can follow any of the following startegies,

1. Sample daily by fixing a time ( either morning, afternoon or the dusk)

2. Sample on the alternate days diurnally.

Choosing the strategy depends upon your objective.

The logic should be that your samples should represent all the seasons significantly with a good sampling number. When you have a good number of samples, applying statistics to it should not be a problem.

Thank you for the valuable suggestions Hema Priymavada.

The fact is I am using Six-stage Andersen cascade impactor for sampling and the duration would be for one year. Considering the fact that I have to observe both diurnal and seasonal variation, I may need to sample all three time periods viz, morning, noon and dusk. Having six plates generated per sample and three times sampling a day, sampling alternate days for multiple sites would generate sample volume too heavy and difficult to maintain if the seasonal variation is to be analyzed too. The question, therefore, lies with what frequency of sampling should I opt out to acknowledge seasonal sampling that would generate sample size optimum for statistical analysis? The constraint doesn't lie with sampling but with the load of biochemical analysis that I will need to perform for microbial identification. Further, I suppose Biosampler SKC is based on impingement method and follows a different course of action for identification.

Thank you