What contributes to the higher bather load in spas?

The correct choice in this scenario focuses on the smaller size of the facility typically found in spas. Spas are generally designed to accommodate fewer people in a more intimate setting, which can lead to a higher concentration of bathers relative to the volume of water available. As a result, when compared to larger swimming pools, the limited space in a spa can create an environment where the bather load appears higher, influencing various factors such as water quality, chemical balance, and the effectiveness of sanitation measures.

While the other options might seem relevant, they do not directly contribute to an increased bather load as effectively. For instance, longer duration of use may raise concerns about water quality or sanitation, but it doesn't inherently indicate a higher number of bathers. Less filtration time factors into water clarity and hygiene but also doesn’t reflect bather capacity directly. Similarly, more frequent cleaning cycles are important for maintenance but don't correlate directly with the number of bathers within the smaller confines of a spa. Thus, the configuration and size of the facility play a significant role in determining the bather load in spas.