You've made a great decision in deciding to wear a helmet while cycling. Now, fitting it properly is the next step to really gaining the intended safety attributes. Cycling helmets are designed to be lightweight, aerodynamic, cooling and safe too.
Today's road and mountain bike helmets are unlike those of the 70's and 80's that were a solid heavy plastic. BMX helmets are still quite thick like the older models, however they are prone to repeated impacts as riders learn new tricks and take bigger and bigger risks to achieve them. But on the road or on the trail, a helmet needs to be light, comfortable and essentially, unnoticeable. This is achievable with the proper selection and set-up.
A standard, modern helmet is made of polystyrene foam. It is created from a mold and can be shaped in many different ways. Many helmets today also contain a plastic or carbon netting within the foam that helps keep the helmet in one piece in the event of an impact. Over the top of the helmet is a very thin plastic shell. This also helps keep the helmet together as well as providing other purposes. Original foam helmets did not have plastic shells or were covered with a lycra cap. However, it became apparent that a plastic shell would allow the helmet to slide when hitting a surface as opposed to just the foam possibly gripping asphalt. The plastic helps prevent possible neck injury. It also allows for unique designs and paint schemes while protecting the foam from general wear and tear.
When choosing a new helmet, it is best to try it on first before buying. If you intend to purchase online, you may wish to stop in at a bike shop first and test a few models. Many different companies manufacture helmets such as, Bell, Giro, Rudy Project, Louis Garneau etc. They also all fit differently. Each person's head is also shaped differently, so it's best to choose a helmet that fits you best and does not have any pressure points. If the helmet feels uncomfortable when you first put it on, one can expect that to be more distracting as you ride your bike also.
An expensive helmet is not necessarily any safer than an inexpensive helmet. All helmets on the market are required to pass certain safety standards by organizations such as Snell, ANSI or CSA. Look inside the helmet and verify that there is a sticker indicating that it has passed one of these certifications. The difference in price is generally a result of the research and development that has gone into the design and production of the helmet. One that is extremely light with many air vents still has had to pass the same safety test as a heavier model with less vents. So it is quite likely that the manufacturer has had to invest more time and money in developing the lighter model. Another option that would increase the price is if the helmet has an additional retention system at the back. This is usually a plastic piece that wraps around the back of your head and offers additional, adjustable support and comfort by way of a dial. This will increase the price of the helmet, but in my opinion, it is well worth the expense. You may also be paying for a snazzier plastic shell paint job. So decide what you can afford and what options you would like. But rest assured that your safety is not in question if it meets the aforementioned standards.
With regards to children's helmets, you may notice that there are a couple of varieties to choose from. There is the older style full coverage helmet that resembles the hard shell plastic BMX helmets, and the more modern style. The older style does have additional protection for the back of the head. This isn't really a concern for adults who ride road or mountain bikes as a blow to the back of the head is quite unlikely. But for a young child, you may want to take this into consideration if they are on a bicycle that could tip backwards. Also, keep in mind that if you are taking up any sport where a fall backwards is likely, such as rollerblading, you may want to consider the full coverage helmet as well.
The goal when fitting a helmet is to have it rest comfortably on your head in a level position. It is not meant to sit on the back of your head. If you have any forehead exposed, it is sitting too far back. The helmet cannot do its job if it's not placed correctly. Tilt it forward until it is level but not covering your eyes. Then adjust the straps so that they are all reasonably snug when clipped together under your chin. Don't leave the straps so loose such that they hang clear under your throat (also known as hockey helmet). The helmet needs to be snug on your head and not slide or jump around when riding over bumps or down a fast descent. It is best to adjust your straps well in advance of your first ride. It can take some time to get things perfect so don't rush it. Finally, if you have a retention system in the back, tighten it until snug but comfortable.
Most helmet manufacturers also include some extra padding to add if necessary, or different thicknesses to change around. If your helmet is too loose or too tight, consider switching these pads around to achieve the best possible fit. But also consider trying a different size that precludes you from using a lot of extra pads.
What about used helmets? Are they safe to use? A polystyrene helmet is generally intended to be a one-time "use" item, meaning that if it sustains an impact, it may not be able to absorb another as well the next time. Check a used helmet for cracks or large dents. Locating one means that the helmet has probably taken a hit at some point. Don't use this helmet. Buy a new one. If you are unsure about a scratch or dent, take the helmet into a bike shop and ask them for their opinion. Also, keep in mind that many manufacturers offer a crash replacement warranty whereby you can get a new helmet at a reduced cost. So send in the warranty card when you buy a new one.
Helmets today are much more comfortable, breathable and aerodynamic than their predecessors. So keep safe and wear yours on every ride, every time.