New research sheds a whole new light on what happens when you flush the toilet. On 8th December, 2022 a group of CU Boulder engineers ran an experiment with bright green lasers and cameras to show how tiny water droplets that can't be seen with the naked eye are quickly released into the air when a public restroom toilet with no lid is flushed. Aerosolized particles like these are known to carry pathogens and could put people who use public bathrooms at risk of getting sick. This visualisation method, on the other hand, gives plumbing and public health experts a consistent way to test better plumbing design, disinfection, and ventilation strategies to reduce the risk of pathogen exposure in public restrooms. 

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- John Crimaldi, lead author on the study conducted by NIOSH (National Institute for Occupational Safety and Health) and professor of civil, environmental, and architectural engineering.

Many people think that closing the toilet lid will save them from facing toilet plumes.

The study by University of Colorado Boulder showed how these airborne particles fire out rapidly, at a rate of 6.6 feet (2 metres) per second, and reach 4.9 feet (1.5 metres) above the toilet in 8 seconds. The biggest droplets usually land on the ground within a matter of seconds, but aerosols smaller than 5 microns (one millionth of a metre) can stay in the air for several minutes or longer. 

The study observed contamination of the toilet seat and the underside of the lid and also cultured Salmonella from the air sample.

However, closing the toilet lid does not solve the problem because the seat and the inside of the lid continue to carry the dangerous disease causing bacteria, which poses a health threat to users.

The experiments lays the groundwork for future work that will test different ways to keep people from getting sick from flushing toilets. This could include looking at how the aerosol plumes from new toilet bowl designs or flush valves that change how long or intense the flush cycle lasts change. To this day, there are ways to limit how much people are exposed to toilet plumes. Closing the lid before flushing seems like a good idea. But it doesn’t eliminate bacteria completely, and they continue to live for several days.

• The first experiments were conducted on the bio aerosol content of toilet plumes in the 1950’s.
• In 1975,  Charles P. Gerba's study popularized the finding that diseases can be transmitted by toilet plumes.
• The phrase "toilet plume" has been in use since 1999.