What factor can significantly affect the required amount of chlorine in a pool?

The number of swimmers in the water is a significant factor that affects the required amount of chlorine in a pool because each person introduces organic contaminants such as sweat, skin cells, cosmetics, and other substances into the water. These contaminants can react with chlorine, leading to its depletion as it works to sanitize the pool.

Additionally, an increase in the number of swimmers raises the overall demand for sanitation, necessitating more chlorine to maintain appropriate levels of free chlorine and to ensure safe swimming conditions. As a general rule, for every person that swims in the pool, the chlorine levels may need to be adjusted to keep the water clear and free of harmful bacteria or algae growth, ensuring optimal water quality.

Other factors such as the frequency of pool parties, number of rainstorms, or type of pool cover used could influence water chemistry and pool maintenance, but they do not directly correlate to the immediate demand for chlorine in relation to user activity as closely as the number of swimmers do.