1. Foam Air Filters:
These are the filters that were used in the very beginning of times. Foam is a very dense material and hence can be used quite efficiently as an air filter and it does filter out air quite efficiently. Even today in many bikes like dirt bikes which have to handle a lot of dirt, foam air filters are used. The main reason being they are efficient and most importantly are washable and reusable. They do not cost a lot, but they are quite expensive compared to paper filters, but then again they are reusable by washing and hence are quite value for money in the long run. Some of them even come in an oiled variant so that they can work even better since the oil helps capturing more minute particles in it being sticky.
2. Paper Air Filters:
These are the cheapest and most easily available filters. Many of the modern bikes use this filter because it is cheap to get, easy for maintenance and can be disposed off after a certain amount of kilometers depending upon how dirty it has become. Maintenance of paper filters is very easy because all one needs to do is blow a little pressurized air on it to clear the dust caught in it. The problem with paper filters is that they are not really as efficient in very dirty conditions which could cause issues. Hence they are mostly used on commuters and on bikes which are meant to be used on the highways and regular roads and not for any of the off-roading bikes because the huge dirt particles would easily clog up the system and also some big particle could rupture the paper element and render the filter useless and end up harming the bike.
3. Cotton Air Filters:
These are by the far the best performers and the most expensive in the list. Cotton air filters are actually made of cotton fabric which is strong, breathing material and fine enough to filter the air really well. They are used in all the performance bikes because it allows for the best air filtration and that too at a very rapid rate which allows for better performance of the machines. Maintenance of these filters is not a tough task either since they are washable but one has to make sure not to scrub it. There are many air filter cleaning kits available today which could be used to clean the cotton air filters along with an additive kit to make sure they are cleaned properly and also the oily substance used to clean the filters makes sure that not even a single microscopic particle is let through except for clean particle free air.So these are the main types of air filters. You can find others too in the market claiming high levels of filtration and improved performance but beware and do not fall for any of that since most of them use the same filters and/or combination of these filters to improve performance and efficiency and what not. But one has to remember that air filters are necessary and hence it is important to get them cleaned up at every service and not skimp out on it and replace them whenever necessary.