Here is how to recognize those situations and what to do the moment you do. The EPA explains the dangers of carbon monoxide in its overview of CO and indoor air.
If You Smell Gas
Natural gas has a distinctive sulfur or rotten-egg odor added to it so you can detect leaks. If you smell it anywhere in your home, treat it as a serious emergency, even if the smell seems faint or you are not sure what it is.
Do not flip any light switches, turn off or on any electrical devices, or use your phone inside the house. A single spark from a switch or a phone screen can be enough to ignite gas that has built up in an enclosed space. Do not stop to grab belongings.
Leave every door open as you go, but do not waste time propping them if it slows you down. Get everyone out of the house, including pets if they are immediately with you. Once you are outside and clear of the building, call 911 and your gas utility from a neighbor's phone or from the street. Do not re-enter for any reason until a utility technician or first responder has cleared the building and told you it is safe.
In the Pacific Northwest, where many older homes still have gas furnaces that have not been serviced in years, a cracked heat exchanger or a damaged gas line connection behind a wall can release gas slowly. You may not notice a small leak until the smell has already concentrated. If there is any doubt about what you are smelling, treat it as gas and get out.
If a Carbon Monoxide Alarm Sounds or Someone Has Symptoms
Carbon monoxide is produced by combustion appliances including gas furnaces, boilers, and water heaters. Unlike a gas leak, you cannot smell it. That is what makes it particularly dangerous. Your only reliable warning is a CO detector — and the symptoms it is trying to signal before the alarm goes off: headache, dizziness, nausea, confusion, or a feeling of sudden fatigue that lifts when you go outside.
If your CO alarm sounds, do not stop to investigate the source or reset it. Get everyone out of the house immediately and move to fresh air. Call 911. Tell the dispatcher that your CO alarm is sounding and describe any symptoms people are experiencing. Paramedics and the fire department are equipped to assess exposure and check CO levels in the building. Your gas utility should also be notified.
Do not go back inside until emergency personnel tell you it is safe. Even if symptoms feel mild, CO exposure can escalate quickly and its effects can linger. Let medical professionals evaluate anyone who was inside.
If You Smell Burning or See Sparking Near Your HVAC Equipment
An electrical burning smell near a furnace, air handler, or any part of your HVAC system is a different kind of emergency. It typically means wiring is overheating, a motor is failing, or something is arcing inside the unit. These conditions can cause fires.
If you can safely reach your electrical panel without touching any wet surfaces and without moving through a room where the smell is strong, shut off the breaker for the HVAC system. If you cannot reach the panel safely, or if you see flames or smoke, leave the house and call 911.
Do not restart the system after you shut it off. Call a licensed HVAC technician and describe what you smelled and when. Let them inspect it before anything is powered back on. Electrical faults inside HVAC equipment can start fires inside walls and in unconditioned spaces where they go undetected for a while.
Water Near Electrical Components
Pacific Northwest winters can mean roof leaks, condensation issues, and flooding, and these sometimes put water in contact with electrical panels, junction boxes, or the wiring near HVAC equipment. Water and electricity are an electrocution risk.
If you see standing water near your electrical panel or near any wiring, do not touch anything. If you can access a clearly dry and elevated breaker to shut off power to the affected area, do so. If there is any doubt about whether you can do this safely, call an electrician or your utility. Many utilities will send someone to disconnect power at the meter in situations where there is a safety risk.
The equipment can be replaced or repaired. The priority is always getting everyone out of immediate danger and letting trained responders handle the rest. A medical summary of the risks is at https://en.wikipedia.org/wiki/Carbon_monoxide_poisoning.