It’s well known that leaky, poorly ­designed duct systems cost their owners money and comfort in multiple ways. However, such systems can also make the air inside the houses unsafe to breathe. Indoor-air quality (IAQ) doesn’t get nearly as much atten­tion as other green-building concerns such as energy efficiency, but it should be one of the highest priorities for anyone who breathes air and spends time indoors. I grew up with really bad ­asthma, which was often triggered by indoor pollutants. I know the importance of good IAQ. I’ve also learned a good bit of building science and know that these issues are interconnected.

Ducts in forced-air heating-and-cooling systems do one of two things: They either supply conditioned (heated or cooled) air to the house, or they return house air to be conditioned. Both supply and return ducts connect to the air handler, which is essentially a big fan. Supply ducts connect to the side of the air handler that blows air, so they’re under positive pressure. Between the air handler and the supply ducts is the furnace heat exchanger and/or the air conditioner’s or heat pump’s coils. Return ducts attach to the intake side of the air handler and run under negative pressure. Ideally, ducts should not leak. You also want them sized properly and installed well so that air moves through them efficiently. They should also be commissioned — that is, checked and adjusted at initial startup — by an expert to ensure that the system is balanced. In a balanced system, the same amount of air flows through the supply ducts as goes back through the return ducts.

If your duct system is leaky or unbalanced, air from spaces such as the attic, crawlspace, or garage is likely to find its…