If your home's air pollution problems cannot be completely solved by eliminating and isolating sources of pollutants, you will need to install some kind of air filter. Whether that is a small, portable unit for occasional use or a large, whole-house ventilation system depends on how bad the problem is.

In either case, you will need some way of working out which filters are the most efficient. Unfortunately, there are no government guidelines for reporting efficiency, even though the FDA keeps asking for some. You will therefore have to rely on three disparate sources of information:

• Manufacturers' claims, which can be confusing or misleading.
• Ratings from the American Society of Heating, Refrigeration, and Air Conditioning Engineers ( ASHRAE).
• Ratings from the Association of Home Appliance Manufacturers ( AHAM).

Unfortunately, while both ASHRAE and AHAM label products they have tested, none of the ratings listed above are based on health criteria.

There is one notable exception to this lack of ratings: some filters are good enough to warrant a Class II medical device label from the FDA. These models are both safe and provide a medical benefit, for which they receive a listing with the FDA and a label stating that they have the FDA's Class II approval. Look for the label.

At the most basic level, a filter's efficiency is determined by a combination of how much air can be pushed through it and what percentage of the pollutants are removed. ASHRAE started with just the latter criterion for particulate filters: they invented a 16-point minimum efficiency reporting value (MERV) which measured each tested model's ability to remove pollution particles. The higher the number, the better the filter.

That wasn't really good enough, since particles vary in size, so they extended the system (in 2000) to provide values for different particle sizes. This stops manufacturers making outrageous claims based on a single particle size – claims that are usually based on the largest pollutants which are easiest to trap, and which are the least worrying for health issues.

AHAM covers much of the portable air filter market. Their CADR rating – Clean Air Delivery Rate – measures the amount of air filtered by a unit (in cubic feet per minute) for a specific material. For example, if they rate a model with a CADR of 300 for pollen, the room's pollen pollution levels will be lowered to a level as if 300 cubic feet of pollen-free air were being pumped in every minute. The higher the CADR, the better the filter.

Unfortunately (again), both ASHRAE and AHAM only rate particle filters. They don't cover gas filters at all. There is a good reason for this: gas pollutants are all tiny and of similar size, so it's a lot easier to understand and compare manufacturers' ratings. Particulate pollutants vary so much in size that it's very easy for companies to create marketing materials that sound impressive but actually tell you nothing, or mislead you as to the product's efficiency.