After a person uses a toilet, he usually must reach to the toilet's left side for the device's flush handle. Two main theories and a lesser known assumption surround the purpose of the design that places the flush handle on the left side. The first theory relates to the original design for toilets, while the second theory involves bacteria. The assumption concerns symbolism.
The early design of the flush toilet consisted of a chain on the left side of the tank. Pulling the chain flushed the toilet. The chain was on the left so that a person sitting on a toilet could reach up and flush with the right hand. One theory suggests that this early design with the flush device on the left of the toilet's tank translated to the modern handle flusher, which also is on the left side. Someone sitting on a modern-day toilet can reach behind to flush with the right hand.
This is the more likely of the theories, as recreating the design of a toilet to adapt the flusher to the other side would seem to be unnecessary.
Bacteria may have been a consideration when the flush handle was placed on the left side of the tank in a toilet's design. According to the website WebMD, a flush toilet handle contains 83 bacteria per square inch, the 27th most bacteria-populated spot in a typical home. In theory, having the flush handle on the left side of the tank encourages toilet users to use their left hand -- most likely their non-dominant hand -- to flush. That avoids contaminating the right hand -- the dominant hand in most people -- with bacteria.
A combination of tradition and bacteria creates a third theory. In some cultures, including Hinduism, the left hand has been long-held as the unclean hand, mainly due to urination and defecation habits from early times. People would wipe themselves with their left hand, the non-dominant hand, to avoid making their right hand "impure." This impure left hand would clean away impurities, and a handle on the left side can be seen as a symbolic representation of eliminating impurities as well.
While most toilet flush handles are on the left side of the tank, some toilets have the handle on the right side. The Americans with Disabilities Act (ADA) requires flush controls to be mounted on the open side of the water closet. In the amended 2002 edition of its guidelines, the ADA states, "Flush controls for tank-type toilets have a standardized mounting location on the left side of the tank (facing the tank). Tanks can be obtained by special order with controls mounted on the right side." Some manufacturers offer toilets with the flush handle on the right side.