Air purifiers can remove smoke, dust, even pet dander from the air -- if they work.  Chris Olsen has the test results from almost two dozen portable air filters.

North Americans spend more than $500 million a year on air purifiers.

Consumer Reports just tested 21 from companies like Whirlpool, Holmes and Honeywell.

Testers use a special chamber to assess each unit. In one test, they fill the chamber with fine, powdered clay dust. In another test, cigarette smoke is used.

Then testers measure how quickly each air cleaner removes the particles from the air. Some expensive ones had problems.

The $400 Oreck Proshield Plus Air12GU didn't do a good job at cleaning the air and also produced low levels of ozone. That's a respiratory irritant!

"The Ionflow Air Cleaner was about as effective at removing dust and smoke as having no air cleaner at all," John Galeotariore of Consumer Reports said.

In fact, unless you have asthma or allergies, you probably don't need an air cleaner. If you do suffer from a respiratory condition, Consumer Reports says there are steps you can take before buying one.

"If you have respiratory problems, put dust mite covers on your mattress and pillows. Remove any wall-to-wall carpeting, and don't let any pets into your bedroom," Galeotafiore said.

If you still have symptoms after taking these measures, Consumer Reports did find some air purifiers that performed very well.

The $320 Whirlpool AP51030K is top-rated. It's very quiet and has a filter-replacement indicator, which is a very helpful feature.

And keep in mind if you have an air purifier when there is a lot of pollen in the air, smoke from forest fires or other contaminants like smog, you need to clean or replace you filters more often.

With a report from CTV British Columbia's Chris Olsen