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[0] Combustion bi-products [2] Gas Detection Meters. Call your local gas utility if you smell or suspect a problem. [3] Regular appliance maintenance, quit smoking, buy a CO detector, isolate mechanical room [1] 
Faulty gas appliances, cigarette smoke, improper ventilation.
Air – What the Nose Knows | Healthy Home Consultants | Paula Baker-Laporte, FAIA - Healthy Home Design
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