Certain plants are excellent air cleaners. NASA did a study to determine which particular species of plants are best for removing certain toxins from the air:http://en.wikipedia.org/wiki/NASA_Clean_Air_Studyhttp://en.wikipedia.org/wiki/List_of_air-filtering_plants
Some of these are exotic, but others are quite common and easily found. I bought a Gerbera Daisy about a month ago. But it doesn't like the cold temperature in my apartment. I usually keep plenty of windows open, even if it's -20 deg C outside. That's another way to get better-quality air.
Actually, it may not be obvious whether it's better to open all the windows, or close the windows and get lots of plants. It probably depends on the quality of air in your neighbourhood.
I think that before you invest in an air purifier, you should first remove all sources of toxins that are in your home, to the extent that is practically possible. To learn what is toxic, take a look at:www.toxicnation.cawww.hundredyearlie.com
(I can't access this web site right now. Try google cache. It's for Randall Fitzgerald's book "Hundred Year Lie")http://slowdeathbyrubberduck.com/
Warning: reading the above will probably make you depressed. You will learn that toxins are EVERYWHERE in your home. Clothes, coats, carpets, furniture, personal care products, computer equipment, plywood, paint, etc, etc. If you try to change everything at once, you will go crazy. Take it one step at a time. Getting some plants is a good first step.
I used to think that it was unnecessary to worry about toxins in everyday items, until I read the report on toxicnation.ca a few months ago. Now I'm gradually eliminating toxins from my home. It's a long process. I recently bought a mask from icanbreathe.com. (Recommended by Aajonus). I use it when bicycling in street traffic. Eventually, I'll research air purifiers, but it's quite far down on my to-do list.