For full details of Inoculating agar plates and sealing them, please see the answer posted on 03/06/2015
Further information on the gas permeability data for Parafilm M indicates that when used as a single layer it will allow sufficient oxygen exchange to promote the growth of aerobic microorganisms and inhibit the growth of potential anaerobes. Petri dishes should only be wrapped with a single layer and a small overlap of Parafilm during incubation to allow adequate gas exchange. Wrapping Parafilm around a Petri dish more than once should be avoided and will be sufficient to stop any gas exchange creating an undesirable anaerobic environment. (1)
The product information sheet states the gas permeability (rates corrected to 760 mm pressure) to be:
- Oxygen: 150 cc/m2 /24 hours
- Carbon dioxide: 400 cc/m2 /24 hours (both at about 22.8 °C, 50% relative humidity (R.H.)
- Water vapour: 0.88 g/m2 /24 hours (37.8 °C and 90% R.H. vs desiccant).
- Tilbrook, Dr Peta. 2015. Technical Services Manager, Department of Environment and Agriculture, School of Science, Curtin University, WA. Personal communication.
 Sigma-Aldrich, Product Information – Parafilm® M, Sigma-Aldrich website https://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Sigma/Product_Information_Sheet/1/p7543pis.pdf (Accessed June 2015)