You might have lived with dangerous conditions in your home for years.  And maybe its never been an issue.  It’s also possible that it has been but you didn’t realize it.  For example, poor air quality in the home is a major cause of asthma.  Similarly, frequent headaches can be from elevated Carbon Monoxide in your home.

So if you plan to make your home more energy efficient, especially if you make it more air-tight, then you want to make sure you aren’t going to make worse an already dangerous situation.

Carbon Monoxide is called the silent, deadly killer because it is invisible and odorless.  If you have any natural gas, propane, kerosene or oil-fired heating units, make sure they are running well.  Unvented space heaters are NEVER safe to have in your home.  You should probably not do any air sealing if you intend to keep unvented space heaters in your home.

Other major safety hazards in the home you should consider are mold and electrical hazards such as exposed wires or live knob-and-tube wiring.

A very clear sign of spillage. Call an HVAC professional if you see this.
Knob and tube wiring. If you have this in your attic, don't touch it until you're sure it isn't live
Likely sign of spillage. Call your HVAC professional to check it out.
Poorly connected flue means toxic gases are staying in your home
Unvented space heaters produce Carbon Monoxide and other toxins that stay in your home
combustible materials too close to the water heater exhaust. Note the burned areas.
Water damage from a roof leak could lead to mold in your home
water heater flue should not be attached directly to the water heater. Also, the flue is in contact with the drywall above