In hot water: if you're treating your spa water the same as your pool, you re asking for trouble.(polluted water)

When it comes to aquatic venues, spas have gotten a bad name. In fact, more than half of them--57 percent to be exact--may be unsafe, according to the Centers for Disease Control, which inspected more than 5,000 spas across the country in 2004. The biggest violation? Poor water quality.

Clearly, the industry needs to do better.

A good place to start is to understand one basic tenet: Pool and spa water must be cared for in different ways. Spas operate at higher temperatures, and they are much smaller. These two facts change everything. That is why the Association of Pool & Spa Professionals has established separate standards for spas and pools.

The two big differences-higher temperature and smaller volume--cause a number of other differences that need to be taken into consideration.

In general, pools operate between 76 to 86 degrees (25 to 30 degrees Celsius), while spas fall in a range of 96 to 104 degrees (36 to 40 degrees Celsius). This difference in temperature changes the water chemistry in important ways.

Chemical reactions rake place much faster in spas than in pools. For every 18 degree (10 degrees Celsius) increase, the chemical reactions proceed twice as fast. For instance, a spa at 102 degrees allows chemical reactions to happen in half the time of a pool at 84 degrees. Any chemical adjustments occur more quickly as well. The water comes to equilibrium sooner, and water treatment can be completed in a shorter period of time.

The water in a spa also evaporates at a high rate due to high water temperature, rapid water circulation and aeration. As the water evaporates, spa operators must add makeup (fill) water to refresh the system. Any water that evaporates is pure; basically pure water leaves behind everything else--the stuff that we call total dissolved solids or TDS. Makeup water also contains minerals, salts and other things that increase the total dissolved solids, so adding fresh water is increasing the TDS level as well. High levels of TDS decrease the effectiveness of some chemicals and may cause cloudy water.