Don’t worry so much about the vents. You’ll find that you will always be wishing you had more ventilation anyway .
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Some other tips for keeping your head cool on hot days. Wet bandanas are perfect for keeping you cool on short-ish rides (they dry up after 30min – 1 hour). Use a skull cap / face cover and soak it in water. Aside from keeping you cool it also helps keep your helmet from smelling like a jock strap since you can wash it unlike your helmet liner.
A note about D.O.T. approval. They leave the testing up to the industry and do not even require that manufactuers send them the results. There is virtually no government oversight for DOT approval. The only real time that DOT requests test results or preforms them is when there is a lawsuit or legal problem. While this doesn’t mean that every manufactuer is putting out poor helmets just because they only have a DOT sticker it does mean that every now and then some poor quality helmet that can’t protect your bucket will slip through with a DOT sticker on it. So, basically ignore the DOT and look for the SnellM2005(Soon to be SnellM2010) approval. They actively test all helmets before they get the snell sticker. Also even after the first test they randomly pull samples of helmets they’ve approved from store shelves and test them as long as the helmet is still being made and sold.