Back in the 1980’s a British university study found that the larger the front vents, the cooler the helmet. More recent research indicates that for optimal cooling the air also needs to be channeled to the exits, so the interior channels are important to taking in the most air possible through the front vents. In some designs air may even prematurely exit through the side or top vents before performing its cooling role, and researchers have covered some vents and actually improved cooling performance. Some air flows under the edges of the helmet if you have not sealed the edge against your head with fit pads. In short, the interaction of the vents and channels in directing the best cooling flow over your head is not simple and is probably not optimal in many helmets.
HOW TO IMPROVE YOUR COOLING
Cooling can be enhanced on almost any helmet by careful placement of fitting pads to permit air channels around the front and sides. Many riders find that they need a sweat band to keep their eyes clear, and that the foam sweatbands in the front of most helmets do not provide adequate sweat control. That forces you to use a separate cloth sweatband that wraps entirely around the head. The cloth keeps cooling air from reaching your head. (The advantage is that in winter you just swap the sweatband for an ear band without changing the fit.) If you can get along without a sweatband, your head will be cooler. Some may need only a half sweatband with an elastic piece around the back. So pay some attention to the front pad of the helmet and try to find one that is adequate for your level of sweating.
STANDARDS AND TESTING
In short, the test methods developed to date leave a lot to be desired. That does not mean that precise methods that are repeatable between labs with a small margin of error are not attainable, but nobody has worked up a test method yet with that in mind. As a result, it would be very difficult to add this type of test to a bicycle helmet standard at the present state of development. It would, however, be possible to conduct tests for consumer magazines where the only outcome is ranking the helmets by ventilation.