A pool that is 25 metres long, 12 metres wide, with an average depth of 1.5 metres will hold 450 cubic metres of water. If there are, say, 30 people in the pool, each of them will have 15 cubic metres of water each.
Contrast this situation with a spa pool. A spa will only hold about 3 – 10 cubic metres of water, depending on the type. Let’s say we have a spa pool that holds 5 cubic metres and has 10 people in it. Each person now has only half of one cubic metre of water each.
Even though there are more people in the swimming pool (and therefore more total pollution), the relative pollution is higher in the spa due to the fact that as a percentage of volume, the pollution levels are higher. The spa is said to have higher relative pollution levels.
Spa pools are not the only types of pool that suffer from high relative pollution. Any pool that has an unfavourable pollution to water ratio will also have high relative pollution. Examples are: