Portable smoke co2 alarms.

Do you mean carbon monoxide? CO2 is an industrial gas; CO is a combustion byproduct.

First time I’ve seen the topic come up. Could be an issue in your carry on.

Depends where you’re going, I suppose. All hotels and B&Bs I stayed in while in Scotland were equipped with central smokes so, no, I would not consider this a travel necessity.

I have been to Italy, Greece, Rio, Buenos Aires, and North America.

Never saw a need to bring my own.

Really depends where you are going, as to the need to bring one.

Are you asking about some type of smoke and carbon monoxide alarm? Hard to combine into one given that for best results the smoke alarm should be on the ceiling and CO alarm near the floor. Curious as to why you think something like that would be needed especially in the summer time when heating system are generally not operating?

Airbnb apartment? These are not legally required in numerous countries, thus not on the listing. That should tell you it is of minor safety risk. But if it makes you feel more secure, it will need to be battery operated, of comply with European voltage.

Any battery powered detector can be used as a portable.

You probably meant CO not CO2 alarm. Brand and type? As long as it works it does not matter what brand or type it is. Burning with enough oxygen and you get CO2 which is not considered poisonous.CO2 is heavier than air and sometimes you can find it in caves or old mines. If you are unlucky enough and take a breath of CO2 you die very quickly because your blood get so acidic that is not compatible with life. On the other hand CO can kill you even in very low concentration.CO binding to hemoglobin is 250 times stronger than oxygen.

Assuming you mean carbon monoxide, First Alert used to make one presented as a travel device. I remember giving a friend one for his RV. A quick look at their website shows the model I'd bought years ago has since been discontinued, though. Not entirely sure why. There are some other devices (by First Alert and other companies) that aren't billed as "travel" so much as "portable" or with a battery backup. Can't speak to how easy these would be to use, though - ones sold here would likely be 2 US-pronged to plug in to an outlet, probably not dual voltage; don't know what kind of battery-powered operation they could deliver (most have battery backup in case the power goes out, but if you're regularly using it on batteries-only would they last all night, or would it be chirping at you the whole time perhaps?). Amazon has a few with good reviews from travelers, but I can't see anything by travelers using them in Europe or combining em with adapters and/or converters.

A bit of clarification......

"If you are talking about a smoke detector or smoke alarm remember they all have Americium which is radioactive."

That's only true of detectors that use the ionization principle. There are also photo electric smoke detectors that use a beam of light (typically provided by an LED these days) and don't have a radioactive component, and dual detectors that use both types of detection.

However this discussion would seem to indicate that there won't be a problem in any case - https://www.flyertalk.com/forum/practical-travel-safety-security-issues/1198200-carbon-monoxide-smoke-detector-any-restrictions-carry-checked-baggage.html .

Any battery operated Smoke / CO detector should work, although the home models would be a bit bulky to travel with. These units are typically designed to operate for about a year between battery changes.

Remember that most smoke detectors should be vacuumed occasionally to maintain good function, and CO detectors only last about 10 years, in use or not.

However this discussion would seem to indicate that there won't be a problem in any case - https://www.flyertalk.com/forum/practical-travel-safety-security-issues/1198200-carbon-monoxide-smoke-detector-any-restrictions-carry-checked-baggage.html.

That thread was created in 2011, gotta wonder if anything has changed. The creator of it was apparently spent a considerable amount of time in Moscow, but he decided to buy one there from the Russian equivalent of Home Depot.

To the OP, why do you think you need to carry an alarm? You might be able to purchase one when you arrive at your destination.