[ryanobjc]

A p trap can dry out. In my case the resulting sewer gas introduced 120ppm of CO into that bathroom, which started to dissipate and set off CO alarms on the next floor up.

This is common knowledge among the fire fighters here. They knew what was up once they found it.

So, how could VOC be entering buildings where they’re doing CIPP retrofit? Now you know.

[Scoundreller]

Why was there CO in your sewer?

[serf]

carbon monoxide can be found in trace amounts within septic tank gas accumulations.

http://medcraveonline.com/JACCOA/JACCOA-06-00228.php