The kart helmet is required everywhere you race. Even if it isn't required, you would be crazy not to want to wear one.
No. There are regulations and standards on kart racing helmets that need to be followed. You'll learn about them here.
The Snell Foundation makes safety ratings for helmets. There is also the SFI Foundation. Most places just go by the snell rating for standards, but most of them accept SFI Standards as well. Here are the Snell ratings:
Snell rates their helmets every 5 years for the most part. 2000,2005,2010. There are three different types of snell ratings.
Snell SA 2005 Sticker
The Snell SA rating is mainly for the motorsport of auto racing. It is the only rating with flammability testing.
The Snell M Rating is for motorcycling. It goes through the same vigorous testing as the SA, except without flammability testing and the rollbar multi impact test.
The Snell K helmet was introduced for karting. There are not many of them, since most places in karting allow you to use all three of these snell ratings for your helmet. The K rating does not test for flammability, but does include the rollbar multi impact testing.
The Snell CMR/CMS 2007 is rather new by the Snell Foundation and it is designed for childrens motorsports. Children have smaller heads, so the larger helmets required by normal Snell standards were too big for most children. This is in 2 groups, one for ages 6-11, and the other for 12-15.
The SFI ratings are a little different. Snell does their own testing on helmets, the SFI foundation has the manufacturer do it's own testing and then submit it to SFI for approval. Here are there ratings:
The SFI 24.1 rating is for youth helmets. They have to make them lighter and shaped a little different because of childrens small sizes.
The SFI 31.1 ratings are flame resistant helmets for motorsports. They are comparable to the SA rating of Snell.
The SFI 41.1 rated helmets are the same as the 31.1 rating but without flame resistance testing. These are comparable to the Snell M rating.
Most karting organizations allow all three of these above ratings. They all include the same single impact testing by snell, so they are all safe. If you are in a caged kart, it would make sense to get the K or SA since they have rollbar multi impact testing, as well as flammability testing, since you are strapped into the champ kart.
Those seem like easy enough materials. Now, all you do is wrap the tape measure around your head about an inch above your eyebrows and at the largest part of the back of your head. Write that measurement down. I can't give you an exact size for what your measurement is because each brand of kart racing helmets goes off a different chart for their sizes. Look up the brand you want and find their sizing chart to reveal your correct size.
You want a helmet that is pretty snug brand new. The pads should be firmly on your cheeks, and you should definitely not be able to take it off your head with the chin strap fastened. Most people if left alone will buy a helmet too large. Keep that in mind.
The more you spend, the more comfort and extras you will get. The only way you can see for sure how much more comfortable it would be is to try them on, or read some good reviews that you can trust. Venting is a big deal with helmets, as it's easy for it to fog up when your breathing heavy inside one. The more fancy venting solutions the helmet offers, the more expensive it would be.
There are devices, like the Hans device, that offers great neck protection. Some kart helmets come with the attachements already on the helmet for these types of devices. If it doesn't list it as an extra, it probably isn't there, so look out for them.
This is a lot of information to absorb but it's very important and useful. Your kart helmet is the most important peice of protection in karting.