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Think before you flush or brush

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Biology 103
2002 Second Paper
On Serendip

Think before you flush or brush

Sarah Tan

One of my friends from high school has made a habit of putting toilet seat lids down before she flushes. She started doing this about four years ago when she heard that when toilets are flushed, water droplets are expelled from the toilet bowl into the air, and when they land, other areas of the bathroom get "contaminated" by toilet water. That always amused me, but when I went over to her house, I humored her and followed this personal rule of hers. However, I didn't know—and chances are, she didn't know—just how justified she was in worrying about in what is known as the "aerosol effect" in toilets. My discovery that there is actually a technical term for this phenomenon was the first indication that there might be something scientifically legitimate to it. It seems to have first been brought to light by University of Arizona environmental microbiologist Charles Gerba when he published a scientific article in 1975 describing bacterial and viral aerosols due to toilet flushing (2). He conducted tests by placing pieces of gauze in different locations around the bathroom and measuring the bacterial and viral levels on them after a toilet flush, and his results are more than just a little disturbing.

First is the confirmation of the existence of the aerosol effect, even though it is largely unrecognized. "Droplets are going all over the place—it's like the Fourth of July," said Gerba. "One way to see this is to put a dye in the toilet, flush it, and then hold a piece of paper over it" (8). Indeed, Gerba's studies have shown that the water droplets in an invisible cloud travel six to eight feet out and up, so the areas of the bathroom not directly adjacent the toilet are still contaminated. Walls are obviously affected, and in public or communal bathrooms, the partitions between stalls are definitely coated in the spray mist from the toilet (1). Also, toilet paper will be cleanest when it is enclosed in a plastic or metal casing; after all, it's subject to the same droplets splattering on it, and its proximity to the toilet bowl makes contamination potential obvious. The ceiling is also still contaminated and is in fact a potential problem site because it is often overlooked in the cleaning process. Bacteria cling to ceilings and thrive in the humid environment there; if the situation is left untreated for months or years (as is often the case), odors remain in restrooms that seem to have been to be otherwise thoroughly cleaned (1). The bacterial mist has also been shown to stay in the air for at least two hours after each flush, thus maximizing its chance to float around and spread (2). "The greatest aerosol dispersal occurs not during the initial moments of the flush, but rather once most of the water has already left the bowl," according to Philip Tierno, MD, director of clinical microbiology and diagnostic immunology at New York University Medical Center and Mt. Sinai Medical Center. He therefore advises leaving immediately after flushing to not have the microscopic, airborne mist land on you (4). Worse still is the possibility of getting these airborne particles in the lungs by inhaling them, from which one could easily contract a cough or cold (6).

Obviously, the idea of toilet water being unknowingly distributed around the bathroom is less than appealing, but a study of this sort calls for looking in detail at precisely what microscopic organisms we're dealing with here, even if we don't really want to know. Put rather graphically, it can be summed up as the F3 force: Fecal Fountain Factor, compounded by the favorable temperatures for bacterial propagation in room temperature toilet water (3). Using a more scientific viewpoint, streptococcus, staphylococcus, E. coli and shigella bacteria, hepatitis A virus and the common cold virus are all common inhabitants of public bathrooms, but just because they're all over the place doesn't mean we necessarily get sick. After all, humans carry disease-causing organisms on our bodies all the times, but with healthy immune systems, the quantities in which these organisms exist is not enough to affect us, particularly with a good hand-washing after every restroom visit (4). This begs the question, however, of the number of people who actually wash their hands after going to the toilet, and more importantly, the number who wash their hands effectively. Simply rinsing one's hands under running water for a few seconds without soap, as some people do, is not effective at all. The way to ensure maximum standards of hygiene is to lather your palms, the back of your hands, in between fingers, and under fingernails for 20-30 seconds with soap and hot water; the friction will kill off the bathroom bacteria (6).

Toilet seats have actually been determined to be the least infected place in the bathroom because the environment is too dry to support a large bacterial population (7). In accordance with that theory, the underside of the seat has a higher than average microbial population. The place in a restroom with the highest concentration of microbial colonies in restrooms is, surprisingly, the sink, due in part to accumulations of water where these organisms breed freely after landing their aerial journey. While toilets are obviously not sterile environments, they tend to not be as bad as people think because they receive more attention and are cleaned more often. "If an alien came from space and studied the bacterial counts, he probably would conclude he should wash his hands in your toilet and crap in your sink," Gerba said (2). The alien would almost certainly not put your toothbrush in his mouth because, with its traditional, uncovered spot in the bathroom, it is one of the hotspots for fecal bacteria and germs spewed into the air by the aerosol effect (5). Understandably, the toothbrush with toilet water droplets on it is one of the most retold horror stories to emerge from Gerba's report.

There are also greater implications from the study of the aerosol effect than simple grossness factor. Most obviously, bathrooms should be cleaned even more meticulously than before, with emphasis not just on and around the toilet, but equal emphasis on all areas of the bathroom because all areas are equally affected by the spray. Using the right cleaners is important because all-purpose cleaning solutions are not necessarily antibacterial, whereas most cleaners made specifically for restrooms are referred to as disinfectants or germicidal cleaners (1). Given that the sink area teems with bacteria, one must now be more careful about washing hands properly after walking into the bathroom for any non toilet-related purposes like washing your face and brushing teeth. Using a hair dryer can potentially be problematic in regard to bacteria counts because the effect would be largely the same as hot-air hand dryers, which actually increase the bacteria on hands by 162 percent, as opposed to paper towels, which decrease them by 29 percent (7). If you're still not convinced that bacteria exist in any significant quantities on your hands, consider that kitchen sink actually harbors the most fecal matter in the average home, carried there by unwashed hands after using the bathroom (5). A tablespoon of bleach in a cup of warm water on the offending sink will fix the situation... for the day.

To limit the scope of the aerosol effect, the simplest method is to close the lid on the toilet every time before flushing (5). This would also provide the peace of mind that while you are washing your hands for 30 seconds, microscopic, bacteria-laden water droplet will not be descending upon your person. Unfortunately, most public toilets, including the ones in Bryn Mawr's dorms, don't even have lids for that option. Besides, given the large number of people who have used the toilet before you, it probably wouldn't make much difference. After washing your hands, use a paper towel to turn off the faucet and to open the door to leave, in order to avoid being recontaminated (4). And today, get a new toothbrush and always, always keep it in the medicine cabinet or some other enclosed place after use (2).



(1) Janitorial Resource Center - Dr Klean.

(2) A Straight Dope Classic - Cecil's been asked.

(3) Car Talk's mailbag - People are talking back.

(4) WebMD - What can you catch from restrooms?

(5) Harvard Gazette book review - Overkill, by Kimberly Thompson

(6) When in doubt, Ask Men - What can you catch from (men's) restrooms?

(7) Sean Blair: Writer. Researcher. Editor. - Killer offices.

(8) The Atlantic Monthly - Something in the water.



Comments made prior to 2007
With regards to the artical of putting the toilet seat down I have to say I agree but my partner disagrees. He absolutely refuses to put the lid down before flushing after he urinates. He says there is no harmful bacteria in urine like there is in stools. If there was the urinals in mens toilets wouldn't be made the way they are. He even goes so far to say that some people - ( I can't find how this is spelled but it sounds like avedics ). Please provide me with some scientific evidence that this is incorrect ... Shirley King, 17 October 2006


Kim Quinn's picture

Airtight toilet seat

Here is something I found now I have to find out where to purchase it-

Margaret's picture

UTI's and the toilet and sink

Well, I can't believe I guessed right! About a lot of it. I did not think about putting the toilet lid down every time. But in desperation to rid myself of UTI's I began cleaning the sink, the toilet lid and seat, all doorknobs, handles, and bottles with hydrogen peroxide. I clean the toilet bowl every time with bleach. I even clean the floor! But I did not think about the walls. I also started using paper towels instead of cloth towels to dry my well-scrubbed hands. Just think what must be on a towel after drying hands on it! I only use a bath towel once and take only showers, after which I wash my feet again since I have been standing in what is now a dirty bathtub. It may all sound excessive, and created a LOT of laundry, but I am now free of the UTI's. (And of course I wipe the proper way.) Oh, and I also wash my hands BEFORE as well as AFTER using the toilet.

Thank you so much for this valuable information! I am so happy to have found it. I am allergic to all anti-biotics, and cranberry juice and various other home remedies weren't working. A clean bathroom did.

Thanks again. Margaret

The Anal Retentive's picture

Ick Factor ....

I read this article with increasing horror, as after nearly 60 years, I realised my dear departed mother was right! She ALWAYS put the lids down when toilets not being used; although from memory, never deigned to tell us why when we were kids. As a result, in our mostly one toilet households, the lid was left up unless Mum had 'paid a visit'. Since putting new lids on my three dunnies (Australian slang for the ubiquitous porcelain item, for non Aussie readers), I always keep the lids down, not for hygiene reasons until I read this article but because I changed the boring old plain white lids for fancy pictorial jobs which cost a fortune and I want to be able to see 'em when entering the 'convenience'. However, signs will be going in advising visitors to PLEEZE put the lids down, and why!

As an observation, I wonder how absolutely horrifyingly toxic the bacterial counts from those awful German 'poo shelf' toilets are! Google them if you haven't seen one before - use to non-Germans (and probably even a lot of them these days) is a malodorous assault on the senses and a totally confronting experience! German design at its worst.

Serendip Visitor's picture

First world problems

First world problems...

Art's picture

Although it doesn't make the

Although it doesn't make the problem truly go away, I suppose one could put one of those bleach tablets or other disinfecting agents in their tank - so then you can spray nice, clean sterilized human excrement all over the place. Woo!

Art's picture

No good solution?

This is really disgusting. The more you think about it, it would probably be more sanitary to do one's business outside, where your stuff goes down and stays down with no water to make a further mess with nasty water splashing back all over the place. Rather ironic for our modern "sanitary" society, huh?

And yeah, I'd like to see some official stats on what putting the lid down actually does. As mentioned previously, toilets that haves gaps (every one I've seen in my life) would seem to just change the problem - making stuff shoot out sideways, all over the wall, the toilet paper dispenser, etc. No matter what, it's all pretty hopeless!

Which leads me to ask: Why don't toilet manufacturers address this issue by:

1) making a toilet with a true seal when the lid is down

2) design a toilet that doesn't spray all over the damn place

I'm sure bother are possible. The technology for #2 must certainly exist, albeit it would be more expensive - like something for a "luxury" toilet.

But really, what a piss poor design toilets have then (no pun intended). Again, maybe just better to use something else besides a toilet - lol...

Oh, and this really changed my mind about something else. I've always find it totally DISGUSTING when people don't flush public toilets, but maybe some of these just don't want to be sprayed all over? Perhaps these people are actually some of the cleanest on earth - not the pigs I've always thought them to be!


mv persinger's picture

Close the Lid. Stock Llysol

This is disgusting, but practical info. Emphasize Close the Lid before flushing. Spray all with Lysol after flushing???

Smarten_Up's picture

Another good reason for

Another good reason for putting the toilet lid down?

The stuff that can fall into it!

Hairbrushes and combs, cell phone, TOOTHbrushes, etc, etc.

An open hole to the sewer gathers more than a covered one...

Serendip Visitor's picture

The human immune system would

The human immune system would be ineffective, were it not for bacteria, viruses and more. Just practice good hygiene and you'll survive.

Serendip Visitor's picture

The human immune system would

The human immune system would also be unneccesary if there were no bacteria or viruses. I'd rather not dry my body, brush my teeth, and comb my hair with bits of my own fecal matter. None of the toilets in my college dorm have toilet seat lids, and the toilets are the industrial kind so they really spray when you flush (sometimes you get hit in the face with a drop). It's gross, there are even brown stains on the walls and ceiling. I've been trying to get them to give me a lid.

nunya's picture


bro its easy just read the dang paragraph and use yo inferences

Tuan's picture

Flushing multiple times

What happens when the toilet is full and you need to flush multiple times?
Will contaminated air particles escape once the lid is opened again?

SngnArtist's picture

I'd like to know this too

I am a bit of a germaphobe and would like to know this too. I often need to flush multiple times and always flush with lid down. I clean with an natural orange based cleaner and its a disenfectant etc and spray it in while opening the lid again. I know crazy but makes me feel better that I'm possibly not spreading the aerosol all around me and the room. I've read with each flush the germs get less and less so possible flush a few times then use again? I always thought with the force most would *stick* to the lid and easy to just clean that.

For public bathrooms I use toilet paper as a makeshift lid though I don't usually think about it when I only urinate. Also now carry small bottle with alcohol (that I have to have to keep scissors clean as part of my job as dog show vendor) and spray toilet with it. It's not the idea of getting sick as I have a pretty good immune system, it's just the thought of fecal matter on my person that grosses me out lol.

Also I wonder why fecal matter spreads in the air when it's not diarrhea that would make more sense to me. Another thought we can put a man on the moon etc but we can't manage to make toilets that don't spray contaminants into the air?

Serendip Visitor's picture

Closing the lid isn't much of a use.

Actually what happens when you close the lid is that the cloud squirts out from the sides at a force which is worse for you if you happen to be standing on the floor.

Counselor Counselet's picture

Evaporated toilet water....

With all due respect, the lid down is the best way to control having the moisture evaporate into the air. I know that sickness of different sources can materialize due to the germs you can breath from evaporated toilet bowl water. It is sad that not enough people are aware of the danger. Think about it.... It take one second to put the lid down and two seconds to put it up. Be safe and a caring person and love your family and friends alot. Please start practicing in order to improve your health.

Serendip Visitor's picture

Also keep in mind, never

Also keep in mind, never flush while sitting on the toilet as this kind leed to UTIs!

sirhillary's picture

Toilet Water SARs Study

Thanks for the article, we try to inform our customers about this in the UK because we sell an effective solution based upon the use of ultra violet C light that kills any bacteria, virus 24/7 in these environments.

Please read the Amoy Apartments case study about a SARs outbreak that was started by the infected individual flushing the toilet in his apartment resulting in the SARs virus travelling on the prevailing winds infecting 321 and killing 42 people -

sally's picture

NO! I'm never going to leave

NO! I'm never going to leave my toilet lid up again after reading this. I'm also throwing out my tooth brush too, which is a bit of a pain in the ass since I just wasted money on a new malibu bright brush :/

Vulcanbabe's picture

Think before you Flush

Ummm what about those fluffy towels, hair brushes,make up and skin care,tissues,babies bath toys,shaving brushes,bath and toilet mats,the door handles,candles,extra toilet rolls on a holder and who has books/magazines in a book rack sitting next to their toilet for a quiet read, what else do you have in your ensuit/bathroom!!!!

Heck just give me a bidet!!!

Phoenix Portable Toilets's picture

bodet... pumper

As a man that pumps phoenix portable toilets for a living, I've had my fair share of surprises in the pot. We carry flushing portables that have come back with live baby chickens, prosthetic limbs and glass eyeballs. Portable bodet, that's a great idea.

Anonymous's picture

I thought this is one of the

I thought this is one of the most interesting articles I have read in a long time. I have always put the lid down on the toilet for sanitary reasons and because the toilet looks much better with the lid down (manufactures put the lid there for a reason). Looking into someones toilet when you walk into a bathroom is not a pleasant site. My husband accidentally left the lid up, flushed the toilet, left the room. I promptly threw out our grandson's toothbrush and bought another one. I could not imagine him brushing his teeth with a contaminated toothbrush yet people do it every day to themselvs and their children. Put the lid down and stay healthier!

Terry B's picture

Selective choice of facts

While the author cited Charles Gerba, she neglected to mention that of the 15 areas he studied, the toilet was the least germ-infested.

To quote Cecil Adams, also cited but ignored (though she felt entitled to rip off the illustration from this article uncredited):

As Professor Gerba's research would later determine, however, the bathroom was hardly the most dangerous part of the house, microbe-wise. The real pesthole: the kitchen sponge or dishcloth, where fecal coliform bacteria from raw meat and such could fester in a damp, nurturing (for a germ) environment. Next came the kitchen sink, the bathroom sink, and the kitchen faucet handle. The toilet seat was the least contaminated of 15 household locales studied. "If an alien came from space and studied the bacterial counts," the professor says, "he probably would conclude he should wash his hands in your toilet and crap in your sink."

"Women's public restrooms contained twice as much fecal bacteria as men's, probably because the women were accompanied by sanitary napkins, grimy small children, and babies in need of a change."

Anonymous's picture

fecal particles

first i'll start off by mentioning i have OCD but in reading all these scientific articles I realize my fear of germs is fact based! My question is don't fecal "particles" also fly around your bathroom when you are wiping by means of toilet paper lint? and if so how far do they go?

Dawn's picture

Great Article

We’d always kept the lids down after use at our house for cleanliness and aesthetic reasons. Then I read an article on ABC Science about how airborne particles travel after flushing (Ew!). Ever since then I’ve put the lid down even before flushing.

Here's a video of airborne particles during a flush:

Serendip Visitor's picture


j. barker and m. v. jones. 2005. To determine the level of aerosol formation and fallout within a toilet cubicle after flushing a toilet contaminated with indicator organisms at levels required to mimic pathogen shedding during infectious diarrhoea. A semisolid agar carrier containing either Serratia marcesens or MS2 bacteriophage was used to contaminate the sidewalls and bowl water of a domestic toilet to mimic the effects of soiling after an episode of acute diarrhoea. Viable counts were used to compare the numbers of Serratia adhering to the porcelain surfaces and those present in the bowl water before and after flushing the toilet. Air sampling and settle plates were used to determine the presence of bacteria or virus-laden aerosols within the toilet cubicle. After seeding there was a high level of contamination on the porcelain surfaces both under the rim and on the sides of the bowl. After a single flush there was a reduction of 2·0–3·0 log cycles cm−2 for surface attached organisms. The number of micro-organisms in the bowl water was reduced by 2·0–3·0 log cycles ml−1 after the first flush and following a second flush, a further reduction of c. 2·0 log cycles ml−1 was achieved. Micro-organisms in the air were at the highest level immediately after the first flush (mean values, 1370 CFU m−3 for Serratia and 2420 PFU m−3 for MS2 page). Sequential flushing resulted in further distribution of micro-organisms into the air although the numbers declined after each flush. Serratia adhering to the sidewalls, as well as free-floating organisms in the toilet water, were responsible for the formation of bacterial aerosols. Although a single flush reduced the level of micro-organisms in the toilet bowl water when contaminated at concentrations reflecting pathogen shedding, large numbers of micro-organisms persisted on the toilet bowl surface and in the bowl water which were disseminated into the air by further flushes. Many individuals may be unaware of the risk of air-borne dissemination of microbes when flushing the toilet and the consequent surface contamination that may spread infection within the household, via direct surface-to-hand-to mouth contact. Some enteric viruses could persist in the air after toilet flushing and infection may be acquired after inhalation and swallowing. [ABSTRACT FROM AUTHOR]

Serendip Visitor's picture

use recycled water (RW) for toilet flushing

Several new utilities are proposing to reuse recycled water (RW)/grey water for toilet flushing.
Little information is available on the scale of the risks associated with this type of end-use
I would like to develop
1. an assessment programme to establish the scale of the risks associated with flushing toilets with RW.
2. Establishing risk (Risk is a function of likelihood & consequence)
3. Sampling techniques
a. Microbiological & chemical contaminants of concerns
b. Assessing exposure
4. Establishing a robust data set

Can you help if you have done any research on the above topics.

Thank you in anticipation. I look forward to hearing from you.

Matthew's picture

Aerosol Effect when Lid is Down

While I too agree that the toilet seat lid should be closed before flushing, how much spray still gets out when it's closed because of the gaps between the bowl and the seat and the seat and the lid. I'd love to see this tested.

Anonymous's picture

toliet seat

It is just the healthy thing to do. I'm told" I have always left the seat up. My whole life. Also the bathroom was not next to the return air system of the the situation IS.... I am admonished for putting the seat down.? Any other suggestions how to convince it is so simple."Put the seat Down"">>>

Anonymous's picture

I have worked in the toilet

I have worked in the toilet repair business for a number of years now and have seen tons of statistics about the airborn bacteria when a toilet is flushed, it isn't good. Closing the seat reduces that bacteria by about eighty percent which is a considerable difference.

If you have trouble remembering to close the lid, maybe you should purchase an auto closing lid, they are beginning to become more common these days.

Steve's picture

Aerosol effect, hand dryers

The first page I linked to confirms many of this article's suspicions but not to as great of an effect as this article implies. The second page (also mentioned at is a claim that hand dryers are *not* really a bacterial concern.

Amy's picture


In response to Shirley's question as to whether or not we should close the toilet lid before flushing "clean" urine. Tell your partner he must admit that he knows there is fecal matter from previous toilet use still sitting in the toilet bowl. If that doesn't convince him, tell him bacteria such as E. coli can live in "clean" urine(see bladder infections). Blood can also be found in "clean" urine. Even if urine really was harmless, men's bathrooms also have toilets without lids: when flushed, that bacteria flies to the urinals and then flies off the urinals when those are flushed. Hopefully, someday, there will be a law requiring that public places add lids...statistics on colds/infection would decline immensely, if people actually use them.

 Frances's picture

toilet contamination

As a nurse, I will tell her partner that Yes, urine is sterile UNTIL it leaves the body. As it passes the outside of the urethra/ body, contaminants are picked up. He needs to try the dye test........he will be convinced.

CassieP's picture

Toilet Flush Aerosol Effect

Thank you for posting this informative report on what happens when the toilet is flushed! I've been trying to convince my kids of this for years.

Scott Timothy Mullenix's picture

Some of this information

Some of this information that I read from this article looks false and looks as if a non-Christian was in on some of it.

sometimes a tiny fecal matter remains in a toilet bowel.

At the kitchen and bathroom sink, leave Comet on it. Other than fungi with it, don't rub it off. It continues to sanitize those areas.

Bathing is more practical than showering.

Serendip Visitor's picture

While you have mastered

While you have mastered translating words into english you have yet to comprehend how to convey meaning. Perhaps your babblings are a reflection of your state of mind.

Serendip Visitor's picture

non Christian?

Exactly what does being Christian or non Christian have to do with putting the lid down on a toilet?

Anonymous's picture

Response to Shirley King / Think before you flush or brush

Regarding the toilet lid being down before flushing - regardless of if it was urine or faeces, it should be down. Yes, urine is "clean" but there is likely to be faecal remnants in the bowl prior to urinating. Yes urinals are not covered - but who craps in a urinal???