Air changes required

[Lan123]

At a waste treatment plant below grade dry well where continuous occupancy is not expected, what is the minimum number of air changes required to keep the concentration of gasses such as H2S and CH4 below a level that will not harm the humans?

To be unclassified per NFPA 820, six air changes per hour is required, though, I am not sure whether this rate is going to be adequate for the above purpose.

Thanks in advance

 

[sfallows]

There is not enough information in here to tell you a number. But I can tell you that you need to know how much gas is being produced. How much ventilation is occurring, mechanically or naturally? The volume of the space?