Carbon monoxide can be produced by heaters that run on natural gas. CO is one of the products of the combustion reaction between natural gas and air, along with carbon dioxide and water. Usually, the exhaust from the burning gas flows out of your home, but sometimes leaks occur and then the exhaust is filtered directly into your home. If the house is well sealed, meaning air doesn't leave or come into the house easily, the exhaust will build up, sometimes to toxic levels. This occurs most often in winter months, because people heat their homes more in the winter months than at any other time of year. Fortunately, there are carbon monoxide detectors which can detect CO at the parts per million level, way before the gas reaches toxic levels.

This gives the occupants time to evacuate the house and to call in experts (like the fire department) to take care of the problem. Carbon monoxide is produced whenever gas or oil is burned, especially if it is burning in a low-oxygen environment. Usually, the exhaust from burning gas is diluted in the atmosphere, so it doesn't cause CO poisoning. On the other hand, if your house is heated by an electric heater, you don't have to worry about CO poisoning because the heater does not generate CO gas!

This is a very serious question. Know you're safe. Here's what happens. A well-sealed house traps dangerous CO. The problem during the cold winter months is a fire, generator and heater produce CO. This CO builds up while warming the house. As I said, no worries because it’s easily preventable. Proper air circulation is key. By keeping the chimney open and not running generators indoors, you won't have a CO problem. Also get a CO detector, which you most likely have already. They are fairly common in modern houses.

CO, or carbon monoxide, is a by product of burning things. Typically when you burn something, whatever it is you are burning reacts with oxygen to form carbon dioxide and water vapor. This reaction does not occur perfectly, however, and some of the carbon converts to carbon monoxide rather than “fully converting” to carbon dioxide. This helps answer part of your question- why CO poisoning is more likely to occur during winter.

During winter people are using their furnaces more- burning natural gas (or coal/wood back in the day- but the explanation applies to all of them). If the system ventilation does not function properly, carbon monoxide will be released to the home. If the home is well sealed, then it will be more likely to build up in concentration rather than being released. This is so dangerous because it is invisible and odorless, so there is no way to detect it without a CO detector. During the summer months people are not burning natural gas to heat their homes (eliminating the source of CO) and typically have windows and doors open more often (allowing ventilation of CO if it did somehow form- like from a stove or something).

If there are drafts and open windows, the CO will go out of the house through those spaces. If the house is well sealed, it needs to have a system that purifies the air, to remove CO and also the molds and odors that are trapped in a well-sealed house.

Carbon monoxide is a colorless, odorless gas that can be produced by burning fuels. These fuels can be found in stoves, gas ranges, and heating systems in houses. In well-aired places, the carbon monoxide is dispersed quickly, and because the concentration of this gas is zero or not high, there is no risk to the people inside the house.

During very cold weather, we tend to close up our houses and houses are therefore unlikely to have good airflow. The poor airflow may stop carbon monoxide from dispersing (spreading out), and can lead to CO poisoning for people in the house.

CO poisoning happens when the concentration of CO is really high, leading to not enough oxygen for a person to breathe. It is more likely to occur during the cold winter months in well-sealed homes for two reasons:

(1) CO is often produced by reactions where fossil fuels are burned. In heated homes, this is often done with burning natural gas to generate heat. In winter, we need to heat our homes more so there well be more CO generated.
(2) A well-sealed home prevents the air inside from leaving the house in order to keep the warmth inside. However, when CO is getting produced inside, this means that the concentration of CO will build up in the house.

So, putting these factors together, when a home is trying to generate more heat during cold winter months and is well-sealed so that air can't really escape, there are higher chances that the CO concentration will build up enough to become dangerous for humans, leading to CO poisoning.

Carbon monoxide is a gas that will diffuse into the atmosphere like any other gas. However, if the house is sealed, then it can't escape, so gets concentrated to levels where it is toxic.

There is a risk of producing CO as long as you are burning something. In the winter months, many people use furnaces to heat their homes (for example, burning natural gas to heat air which is then circulated around the house), and CO can be produced while doing so. Because of the cold, people also seal their house (e.g., closed windows), making it easier for CO from the furnace to build up to dangerous levels inside the house. This is why it's very important to have functioning CO detectors.