@merpreb When I realized that the author was not really qualified to be answering this question I did a little googling and found info on the NCBI (The National Center for Biotechnology Information) website:
https://www.ncbi.nlm.nih.gov/pubmed/23514282
Basically it says,
"PRACTITIONER SUMMARY:
Carbon dioxide (CO2) rebreathing in respiratory protective devices (RPDs) has been highlighted as a key concern regarding respirator use. However, the problem is relatively under researched. This paper presents novel findings on the impact of phonic respiration (breathing during speech) and CO2 concentrations in RPDs."
There is more in the actual article. I find this to be a very reliable and legitimate site for health information.

I personally have been wearing facemasks when flying ever since my transplant and have not had a breathing problem. I do find them very annoying though. Another consideration that I have read in the past, is if you have to wear a disposable face mask for more than two hours, you should change it for a new one. In that amount of time the bacteria that have accumulated on the mask make it ineffective.
It really is a complex topic and one that most of us are still confused about, particularly since the initial information said NOT to wear facemasks. Those of us who have had transplants and been advised to wear facemasks may be a bit ahead of those who have never had a need to wear them.
JK