Closing the Toilet Lid Before Flushing Is Important

Medscape Staff

February 14, 2023

Laser lights expose the otherwise invisible aerosol spray that is sent into the air by flushing a toilet with the lid up. The lasers show how pathogens from human waste can spread in public restrooms and can potentially expose people to contagious diseases, according to the results of tests conducted by a group of civil, environmental, and architectural engineers from the University of Colorado Boulder.

What to Know

  • Toilets are designed to efficiently empty the contents of the bowl through a downward motion into the drainpipe, but the force of the flush cycle also creates a fine spray of particles in the air. Those particles easily spread when a lid is left up during flushing.

  • Laser lights illuminate the otherwise invisible aerosol plumes. They show that a typical commercial toilet generates a strong upward jet of air ― velocities exceed 6.6 feet per second ― that can rapidly carry particles up to 5 feet above the bowl within 8 seconds of the start of the flush.

  • Smaller particles that remain suspended in air can expose people to respiratory disease, such as influenza and COVID-19, through inhalation, while larger particles that settle quickly on surfaces can spread intestinal diseases, such as norovirus, through contact with the hands and mouth.

  • Concentrations of pathogens can persist in toilet bowl water contaminated by feces after dozens of flushes, but it is still unclear as to the risk of transmission through toilet aerosol plumes.

  • Since many public restrooms do not have lids that can be closed prior to flushing to reduce human exposure to toilet plumes, ventilation or UV disinfection systems would be important tools to mitigate exposure to aerosol plumes in the bathroom.

This is a summary of the article, "Toilets spew invisible aerosol plumes with every flush – here’s the proof, captured by high-powered lasers," published in The Conversation on December 16, 2022. The full article can be found on

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