Very few people know the true cause of those musty odors you sometimes notice when opening a spa – particularly if you have not used your spa for a while. These odors come from bacteria growing and multiplying on the spa shell ABOVE the water line.
Here’s what’s going on. When you (and others) enter a spa, you displace water causing the water level to rise. When you later leave, the water level recedes leaving a band of water around your spa, approximately 2 – 5” high above the water line. I call this the “tidal zone”. The water in this tidal zone contains bacteria and it’s no longer in contact with spa water, hence no sanitizer is available to kill them. You close your spa cover, leaving a warm, moist environment – just what bacteria love. They start multiplying, generating this musty odor.
Note that most people think these odors come from the cover, and to a limited degree they might. But as your spa cover does not usually come in contact with the spa water, your spa cover is generally not the source of these smells.
