CFM or Cubic Feet per Minute of air movement is a commonly misunderstood topic in kitchen appliances. There is a belief out there, that like most everything else in appliances, bigger is better, right? Well not necessarily. Using a hood with higher cfm (above what you need for your stove) means more air is being pulled out of your kitchen and your home than needed (like in the ducting diagram to the left). Therefore a lot of cooled or heated air is being pulled out your home, which would lead to higher heating / cooling bills.
Also, a situation of negative pressure could also occur when too much air is pulled out of the home and it is not replaced by air from the outside. In today's construction the homes are becoming more and more air tight and when too much air is pulled out of a home, you need to sometimes "make up" for that lost air by pumping outside air into the home. In colder climates this is a huge issue, in most parts of Canada there are laws in place about maximum cfm's before a make up air system has to be installed (typically 300 cfm is the threshold).
When choosing a hood for your cooking surface, if the hood has too much cfm you're not being energy efficient. A higher cfm hood will consume more energy to operate. The noise level or sone level (the noise measurement standard in rangehoods), is also a consideration. More cfm's = more air being pulled through the grease filters, which in turn creates turbulence and extra noise. If you've ever worked in a restaurant kitchen under the pro hoods they use, you understand the noise level.
The bottom line is buy a hood that properly takes care of the contaminants that are produced by your cooking (steam, grease, odor & smoke). The diagram seen below offers a good way to estimate how many cfm's are advised for your kitchen. Take a kitchen that is 10' high (Z) by 10' wide (X) by 10' long (Y) = equals 1,000 cubic feet. If you have a 500 cfm rangehood in this kitchen, in 2 minutes you will have completely exchanged all the air out of the kitchen (or 30 exchanges in an hour). The National Kitchen and Bath Association (NKBA), recommends 8-15 air exchanges in an hour for proper ventilation, so by this example, we're double the recommended level.
Another good rule of thumb is that 600 cfm is a good round number for use with a gas, all burners (no grill/griddle) cooking surface. Anything more substantial (like a 48" pro gas stove with 6 burners and a grill), then higher cfm's are advised, say 1,000 or above). Anything less than a gas all burner stove, requires less cfm's. An induction, ceran, coil electric cooktop would only need maybe 300 cfm's above it. So there you have it, an introduction to cfm's, and hopefully you can feel comfortable like the person below that your home is being vented properly!