Perhaps the best air purifiers are those from the IQAir line. These units are widely used in hospitals to eliminate mold, viruses, and bacteria. In fact, the Hong Kong Hospital Authority chose the IQAir Health Pro as the only acceptable air purifier during the SARS outbreak. The IQAir Health Pro uses a carbon pre-filter, a gas media absorber, and a HEPA filter. Its efficient design prevents already-cleaned air from immediately reentering the intake duct. It filters air up to 50% faster than most other purifiers, and at almost a 100% filtering rate. Being the best, it is also the most expensive, often costing up to $1,600.

Another hospital-grade air purifier brand is Clarifier. Their three models can all be had for under a grand. They include high carbon content for superior odor reduction, strong HEPA efficiency, and an ultraviolet light for killing off mold trapped in the HEPA filter.

If you want a quiet machine without sacrificing too much efficacy, buy a Blueair or Electrolux model for $350 to $600. The Blueair 601 controls pollen, dust mites, mold, pet dander, and general indoor pollution in a range of up to 679 square feet. The Electrolux 510A is a very quiet and economical unit that includes a filter change indicator light.

If you are only looking to spend under $200, you have plenty of options. Hamilton Beach, for example, often includes ultraviolet light in their machines to filter out viruses and bacteria, and the HB Ultra Digital Air Purifier includes a sensor that monitors and shows the air quality on the control panel.

You can also buy good air filters for cars and very small spaces. Roomaid uses a true HEPA filter and only costs $130, while Hamilton Beach offers one with a 3M filter for just $70.