<P> Acesulfame potassium has been used to estimate how much urine is discharged by swimmers into a pool . In a Canadian study it was estimated that swimmers had released 75 litres of urine into a large pool that had about 830,000 litres of water and was a third of the size of an olympic pool . Hot tubs were found to have higher readings of the marker . While urine itself is sterile, its degradation products may lead to asthma . </P> <P> Swimming pool heating costs can be significantly reduced by using a pool cover . Use of a pool cover also can help reduce the amount of chemicals (chlorine, etc .) required by the pool . Outdoor pools gain heat from the sun, absorbing 75--85% of the solar energy striking the pool surface . Though a cover decreases the total amount of solar heat absorbed by the pool, the cover eliminates heat loss due to evaporation and reduces heat loss at night through its insulating properties . Most swimming pool heat loss is through evaporation . </P> <P> The heating effectiveness of a cover depends on type . A transparent bubble cover is the most effective, as it allows the largest amount of solar flux into the pool itself . Thermal bubble covers are lightweight UV - stabilized floating covers designed to minimize heat loss on heated swimming pools . Typically they are only fitted in spring and fall (autumn) when the temperature difference between pool water and air temperature is greatest . They raise the temperature of a pool by around 20 ° Fahrenheit, or 11 ° Celsius, after being on the pool for a week . Bubble covers are typically applied and removed by being rolled up on a device fitted to one side of the pool (see illustration). Covers fall apart after four or five years due to sun exposure, overheating in the sun while off the pool, and chlorine attacking the plastic . Bubble covers should be removed during super chlorination . </P> <P> A vinyl cover absorbs more sunlight directly, allowing temperature to rise faster, but ultimately prevents the pool from reaching as high a temperature as a clear cover . Vinyl covers consist of a heavier material and have a longer life expectancy than bubble covers . Insulated vinyl covers are also available with a thin layer of flexible insulation sandwiched between two layers of vinyl . These covers are mandatory to be fitted to all pools in areas of Australia that have experienced drought since 2006 . This is an effort to conserve water, as much water evaporates and transpires . </P>

When was the first filtration system for swimming pools introduced