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There are combined CO/NG detectors, very cheap. The best NG detector is your nose. Your air filter will not filter either of these gases, they will pass through. If you have an electrostatic air filter it might set NG aflame .... that would certainly alert you to it. The stuff in your air filter could be grey, black, brown, white ... it really depends ...


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Natural gas has a chemical called Mercaptan the human nose can detect this at ~1 part per billion so probably not that. Excess carbon dioxide the stuff you exhale can cause your symptoms and newer tightly sealed homes can allow the carbon dioxide to build up. You may have make up air on your furnace a small amount drawn in from outside. Because of dirty air ...


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