There are many brands and many types of air cleaners on the market. Of them all the best clearly is IQAir. These units are made in Switzerland to very high standards. To understand how to choose an air filter first lets consider the basic types. The two main types are electronic air filters and mechanical HEPA filters. Electronic filters work by charging small particle such as dust and pollen and attracting them to collector plates. These work reasonably well at the beginning but as soon as particles start collecting on the plates their efficiency reduces tremendously. In order for them to work reasonably well you need to clean the many collector plates constantly. Doing so is both a chore, and often as you clean them the dust, pollen and other particles escape back into the air. Also electronic air cleaners, to a greater or lesser extent, produce ozone. Ozone is a lung irritant. It is especially a bad idea for anyone with asthma. The American Lung Association advises people to avoid air cleaners which produce ozone.
HEPA filters are the most effective method of removing particles from the air. These filters were originally designed for atomic labs to make sure that radioactive particles did not escape from the labs. They are extremely effective at removing all particles from large to small. There are many HEPA filters on the market. I personally have used and rejected many of them over the years. Some, including the most common ones found in the stores, suffer from two problems- one inferior fans that do not last and low efficiency. By low efficiency I mean that while they remove some of the particles in the air- they do not remove enough. Most manufacturers state that they use HEPA filters which are 99.97% efficient, which is true. What they don’t say is that while the filters are highly efficient- there units are not. These are due to poor design- such as air escaping past the filters and cleaned air being sucked back into the air intake. Most of these units average 60-80% efficiency. At an average of 70% efficiency- if there are 2 million particle in a cubic foot of air (an average amount) then 2 million go in and 600,000 come out. At these rates of efficiency the units help clean the air to some degree, but not enough.