Closing the lid when you flush is a reasonable habit, but the research is mixed. Flushing puts droplets in the air, and a hospital study found a lidless toilet released C. difficile. A University of Arizona test found virus on surfaces whether the lid was up or down. None of these studies measured illness at home.
Does flushing a toilet spray germs into the air?
Yes, that part is well documented. A 2013 review in the American Journal of Infection Control, by David Johnson and colleagues at the University of Oklahoma, looked at the published research. It found that "potentially infectious aerosols may be produced in substantial quantities during flushing," that aerosolization can continue through multiple flushes, and that some droplets dry into droplet nuclei that stay adrift in the air.
The same review is careful about the health side. Its results say that "no studies have yet clearly demonstrated or refuted toilet plume-related disease transmission, and the significance of the risk remains largely uncharacterized." That review dates from 2013, so read it as a snapshot, not the last word.
### How many droplets does one flush make?
A separate 2013 lab study in Aerosol Science and Technology measured flushes in a mockup water closet. Its authors came from the University of Oklahoma and NIOSH, which is part of the CDC. They counted up to 145,000 droplets per flush, and 95% of the droplets were under 2 micrometers across (abstract: https://pubmed.ncbi.nlm.nih.gov/26635429/).
Higher-energy flushes made more droplets. The high-energy flushometer type produced over three times as many as a pressure-assisted toilet and over 12 times as many as the lowest-energy high-efficiency toilet, even with similar flush volumes. The authors suggest two things at work: splashing makes the large droplets, and bubble bursting makes the fine ones.
What did the lid studies actually find?
Three studies looked at lids directly, and they don't line up neatly.
| Study | What was tested | What it reported |
|---|---|---|
| Best et al., 2012, Journal of Hospital Infection (Leeds, UK) | Flushing hospital toilets without lids, using faecal suspensions of C. difficile | The bacterium was recoverable from air up to 25 cm above the seat, and surfaces were contaminated within 90 minutes. The authors concluded lidless conventional toilets raise the risk and suggested discouraging them where C. difficile infection is common. |
| Goforth et al., University of Arizona | Virus stand-in (bacteriophage MS2) in household and public toilets, flushed with lid up and down | Contamination of restroom surfaces did not depend on lid position. |
| Zhang, Chen and Lai, published online 2025, City University of Hong Kong | Pilot test of a seated toilet, with different gap heights between bowl and seat | Even with the lid closed, aerosols escaped through the gap. Escape rose as the gap grew. |
### What did the hospital study show?
Best's team used fecal suspensions of C. difficile to mimic the bacteria load during illness. The most bacteria came from air sampled right after flushing. Counts then fell 8-fold after 60 minutes and another 3-fold after 90 minutes. The two lidless hospital toilet types they measured emitted a mean of 15 to 47 droplets per flush. Their conclusion is about hospitals, where patients often share toilets. It isn't a finding about a healthy family's bathroom.
### What did the Arizona study show?
The University of Arizona team, in a household toilet, found no difference in surface virus counts between lid up and down. The toilet seat was the most contaminated surface, according to APIC, and the walls had little.
Two details matter here. Two of its authors are affiliated with the Lysol and Dettol research group at Reckitt Benckiser, and it used a virus that is not harmful to people as a stand-in for real ones. The lid could only be tested on the household toilet, since public toilets typically have no lids.
APIC, the association behind the journal, also reports that closing the lid may change the direction of the plume. Floors in front of and to the left of the toilet were more contaminated with the lid closed, and floors to the right were less.
### What did the Hong Kong study show?
It focused on the small gap between the bowl and the seat. The Hong Kong authors found that 92% of the droplets made during flushing were smaller than 1.0 micrometer (abstract: https://pubmed.ncbi.nlm.nih.gov/41086947/). They also reported that escape rose as the gap grew. At a 6 mm gap, the amount escaping, compared with a lid-up toilet, was 24% for E. coli, 49% for P22 and 57% for MS2 (abstract: https://pubmed.ncbi.nlm.nih.gov/41086947/). So the two viruses escaped more easily than E. coli. They call it a pilot investigation, so treat it as a first look.
So should you close the toilet lid or not?
A closed lid is unlikely to hurt, and the Hong Kong pilot found it let through only a fraction of what an open one did. But the Arizona result and the gap findings say it doesn't contain the plume. If you want the lid down, put it down. Just don't treat it as full protection.
If someone in the house has a stomach bug, cleaning matters more than the lid. APIC says cleaning with only a brush left behind substantial contamination, while cleaning the bowl with a disinfectant and brush cut it sharply. The Arizona team reported that a hydrochloric acid product dropped contamination of the bowl water by more than 99.99% (abstract: https://pubmed.ncbi.nlm.nih.gov/38276944/). APIC also says adding disinfectant to the bowl before flushing, or using tank disinfectant dispensers, both reduced contamination. It adds that when one household member is sick, especially with gastroenteritis, regular disinfectant during cleaning or before flushing may help reduce spread.
Follow the product label. And check first whether a tank product is safe for your toilet parts. Our page on cleaning tablets and fabric softener in the toilet tank covers that.
### How well do these findings fit a home bathroom?
Only partly. Each study has limits you should weigh before you change a habit.
- Best's work was in hospital toilets without lids, using a bacterium linked to healthcare settings.
- The Arizona work used a stand-in virus, and only one household toilet could be flushed with the lid both ways.
- The Hong Kong study is a pilot, and it measured how much escaped, not how much made anyone ill.
- The 2013 review says the disease risk from toilet plume was still largely uncharacterized.
### What about public restrooms and hospitals?
Public toilets usually have no lid to close, so the lid question mostly applies at home. APIC reports similar contamination patterns on the public toilet in the Arizona study. Best's team suggested that lidless toilets be discouraged in settings where C. difficile infection is common. Gerba, the Arizona study's senior author, says healthcare settings need to address any possible route of spread, and that regular disinfection matters.
Does the toilet itself change the answer?
A little, going by the 2013 mockup study. Flush energy drove droplet counts, and the lowest-energy toilet made the fewest. That was one lab setup with fluorescent microspheres standing in for microbes, so it's a direction, not a rule. If you're weighing a toilet swap, dual-flush toilets are one place to start. A skirted toilet is easier to wipe down.
If your toilet is due for replacement, HQ Plumbing & Air is licensed and insured and gives free estimates.