Also found this recent article:
And we had many discussions on this on the Hackteria forum:
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another recent and “more open” paper
A low-cost versatile system for continuous real-time respiratory activity measurement as a tool in environmental research - ScienceDirect!
And look i found the paper for download here!
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They use the Sen 0220 NDIR CO2 sensor. 
I also came across this one from 2014, concentrated on indoor CO2, which has a nice description of how two of the diferent types of sensors measure CO2 (chemical, ndir).
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