One portable generator makes as much carbon monoxide as hundreds of cars. Opening the garage door does not answer that.

The open door does not help, because the problem is output rather than airflow

This is the reasoning error that causes most of these deaths, and the CPSC number is what corrects it: one portable generator can produce the same amount of carbon monoxide as hundreds of cars. Opening a garage door is an attempt to solve a ventilation problem, and what you are actually facing is a production-rate problem that no household opening can keep up with. Guidance states it directly, that an open garage door or a cracked window does not move air fast enough to clear it, and that there is no version of indoors that is safe and no run short enough to be safe indoors. CPSC lists the prohibited places explicitly: indoors, garages, basements, crawlspaces, sheds and porches, even with doors and windows open.

Outdoors is not a binary, and a generator against the wall fails the test

The subtlety that catches careful people who did put the machine outside. Exhaust drifts, and as one guide puts it, a unit next to the house feeds CO through soffit vents, window gaps and AC intakes even though it is technically outdoors. So satisfying the word is not the same as satisfying the requirement. CPSC gives the figure as at least twenty feet from the house, with the exhaust directed away from the home and other buildings, and away from a neighbor’s home too so they are not exposed. Twenty feet is farther than it feels, and it is worth pacing out once in daylight so the decision is already made when it matters.

Twenty feet is the floor, and people who do this professionally go further

Worth knowing so that the CPSC minimum is not treated as a target. An electrician quoted in national coverage puts the practical figure at a minimum of thirty feet from the house while running, and is specific that even fifteen feet is not enough distance because wind can blow the odorless, colorless and tasteless gas indoors. That is a practitioner opinion rather than a standard, and it points the same way as every other piece of guidance here, which is that error should be in the direction of more distance. Separately, OSHA recommends three to four feet of clear space on all sides of the unit, which is a different requirement about cooling and airflow around the machine rather than about the house, and both apply at once.

The symptoms arrive too late to be a warning system

Important because it removes the plan most people are unconsciously relying on, which is that they would notice. CPSC describes CO as the invisible killer because it is colorless and odorless, and states that poisoning from portable generators can happen so quickly that exposed people may become unconscious before recognizing the symptoms of nausea, dizziness or weakness. Their full symptom list is headache, dizziness, weakness, nausea, vomiting, sleepiness and confusion, and several of those are indistinguishable from being tired during a stressful power outage. Judgment is impaired by the exposure itself, which means the decision has to be right before anyone is in the room.

Fit CO alarms with batteries, and the reason is the obvious one

CPSC advises installing battery-operated CO alarms or CO alarms with battery backup outside sleeping areas. The battery part is not incidental: a hardwired alarm is not going to help during the event that made you start the generator, because the power is out. That is worth stating plainly since it is exactly the kind of thing that seems fine at installation and turns out to be useless at the only moment it was needed. Fit them, test them, and treat them as a second line rather than a first, because the placement is what is supposed to keep the CO out of the building and the alarm is what tells you the placement failed.

CO shut-off is a feature you can ask for at purchase

A genuinely useful thing to know before buying rather than after, and CPSC recommends it directly: ask retailers for a portable generator with a safety feature to shut off automatically when high CO concentrations are present, and note that some models with CO shut-off also have reduced emissions. That is a purchasing decision rather than an operating one, which makes it the cheapest safety improvement available on this whole subject. It is not a substitute for placement, since a machine that shuts itself off has already allowed a dangerous concentration to build somewhere, but it is a backstop for exactly the human error this page is about.

There are two correct ways to connect, and the common third one is not among them

Guidance is clear on this: the legal way to power a home from a portable generator is through an interlock kit or a transfer switch installed by a licensed electrician, not an extension cord through a cracked window, and never by backfeeding an outlet. The other legitimate approach is simpler, which is running individual appliances on heavy outdoor-rated cords sized for the load, keeping the house wiring out of it entirely. Backfeeding, meaning energizing the house circuits by plugging the generator into a receptacle, is not one of those options. And note the middle clause of that quote, because it is a hazard in its own right: a cord run through a cracked window is precisely the opening CPSC tells you to keep the machine away from.

Keep it dry without enclosing it, which is a real tension

Two requirements that pull against each other, and the wrong resolution is common. Generators should be operated in dry areas and protected from rain, and they must not be enclosed, which rules out the intuitive solution of putting one in a shed or under a tarp draped over the running machine. Purpose-made open-sided covers and canopies exist precisely because of this conflict, and they work by keeping water off while leaving airflow and exhaust dispersal unobstructed. The other half of the same problem is refueling: shut the engine down and let it cool before adding fuel, which on a machine you are relying on during an outage means planning the interruption rather than being surprised by it.