Why would the septic smell be worse inside when it rains or is very cold? Is there some sort of monitor you can get for you house to determine levels of hazard. Such as carbon dioxide alarms etc.

You are getting an inversion from the weather and it is forcing the sewer gasses down toward the ground. The odor is easily detectable but I doubt you would get enough methane gas to measure. I assume you mean Carbon Monoxide, not Dioxide. Even in relatively high concentrations, Methane is not a poison, it displaces O2. Fairly high LEL. FLAMMABLE LIMITS (in air by
Volume, %):
Lower (LEL): 5.0%
Upper (UEL): 15.0%
So that is not an issue.

Yes, sewer / septic fumes are health hazard.

Usually, you can smell sewer gases before the rain ( low pressure ) as sewer gases tend to hang low. That's how we used to predict rains in the City in the old days ...

Milo

The answer is yes. Sewer gas contains toxins that are harmful to your family. Sewer gas also contains methane, which is explosive. It will also rise with heat and descend when it is cold or raining. Does the smell originate in your home or from the septic tank? Back to you, Tom