There have previously been some discussion on CO2 (not CO) monitoring. I came across this yesterday vThings - CO2 Monitor - Dual Beam - WiFi | Indiegogo . Bit pricey but most CO2 meters are 
I suspect this could be persuaded to link directly to EmonCMS.
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CO2 on EmonCMS using : CO2 Sensor
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Nice, I have been in touch with the creator on indie gogo, -yes it can post to emoncms using the following config:
https://emoncms.org/input/post.json?json={ppm:%s}&apikey=WRITE_APIKEY
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I’m using EmonCMS to record some air quality metrics too
More info on my blog (in French) : Surveiller la qualité de l’air avec du open hardware et du logiciel libre | [macrostuff]
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So I now have my CO2 sensor (first one was sent to Ukraine - replacement sent no bother) from http://vair-monitor.com/ Happily communicating with EmonCMS via (my modified) MQTT process. Surprised how high the CO2 was so I have turned up the MVHR unit 