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Some people do their dishes and leave the suds on, letting it air-dry, or using a towel. Some rinse it off.

I cannot understand the British habit of washing and rinsing dishes in the same dirty water, and drying them without washing off the soap suds. Is this similar to having a bath and not rinsing off the soap? Am I missing something here?

—Letter – The Guardian, UK

Ecological water-saving, dirty towels, or nasty soap taste-arguments aside: I wonder if it is unhealthy to leave the suds on, and as a result, ingest minor traces of soap when drinking/eating.

With most washing-up liquids being relatively newer formulations from petrochemicals, to which long-term exposure is still unknown, residuals cannot be beneficial if ingested. The health effects are probably on the order of second-hand smoking; chronic exposure adds up, incidental exposure is probably innocuous.

—Answer to the above letter – The Guardian, UK

What does scientific research say?

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  • Nevertheless the answers should not focus on the towel too much as this is possibly the main source of recontamination after washing. (would be nice of course to know if so and how much of an effect this has)
    – Baarn
    Commented Aug 27, 2012 at 10:23
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    "...relatively newer formulations from petrochemicals, to which long-term exposure is still unknown, residuals cannot be beneficial if ingested..." is illogical: unknown is clearly inconsistent with cannot
    – Henry
    Commented Aug 27, 2012 at 11:29
  • this is very anecdotal: A physicist once explained to me that the soap would create a thin layer which drains any dirt and itself due to gravity.
    – Anno2001
    Commented Nov 3, 2012 at 19:48
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    I don't know about unhealthy but soapy gravy sure doesn't taste so good.
    – GordonM
    Commented Mar 4, 2015 at 12:58
  • Most British people don't wash their dishes in dirty water, they use running water. It was maybe true a long time age before everyone had running water, but not now.
    – user18902
    Commented Oct 5, 2015 at 14:18

1 Answer 1

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It helps to think about it like laundry. In a washing machine one might assume dirt, oil, and debris comes loose through agitation alone. However, wouldn't the clothing act like a thick, giant filter keeping the oil, dirt, and debris on clothing when the water is drained? If not, wouldn't running a a single rinse cycle clean your clothes?

First you have to understand how soap works. To accomplish that you need to understand what hydrophyllic and hydrophobic compounds are.

Nearly all compounds fall into one of two categories: hydrophilic ('water-loving') and hydrophobic ('water-hating'). Water and anything that will mix with water are hydrophilic. Oil and anything that will mix with oil are hydrophobic. When water and oil are mixed they separate. Hydrophilic and hydrophobic compounds just don't mix.(What is Soap)

The cleansing action of soap is determined by its polar and non-polar structures in conjunction with an application of solubility principles. The long hydrocarbon chain is non-polar and hydrophobic (repelled by water). The "salt" end of the soap molecule is ionic and hydrophilic (water soluble).(What is Soap)

When grease or oil (non-polar hydrocarbons) are mixed with a soap- water solution, the soap molecules work as a bridge between polar water molecules and non-polar oil molecules. Since soap molecules have both properties of non-polar and polar molecules the soap can act as an emulsifier. An emulsifier is capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn't naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed. The soap will form micelles (see below) and trap the fats within the micelle. Since the micelle is soluble in water, it can easily be washed away. - (What is Soap)

The purpose of cleaning dishes is to remove debris, sugars, salt, oils, protiens and fats which would otherwise serve as a medium for bacterial growth. Soap is a surfacant and attaches to the water. So, as @Anno2001 mentioned in his comment without knowing the reason, the action that allows dirty soap mixed with water to slide off an object via gravity is due to polarity,and the hydrophyllic / hyrophobic properties of soap and water.

So, technically no, it isn't unhealthy. Even if detergent with known toxic chemicals were used, it still wouldn't adhere to the surface and be consumed later.

Bringing it back to the washing machine analogy, a measure is given for the proper amount of detergent to use. The same knowledge applies to dish soap. However, common sense needs to be applied, since soap to water ratios or instructions aren't on given for dish soap.* Experience and tactility tells us when we've used too much soap.

This fact can be peer-reviewed and replicated by enough people using a search engine "how to use dish soap" or "what are the instructions for dish soap."

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    This is not an answer to the question, which was "I wonder if it is unhealthy to leave the suds on, and as a result, ingest minor traces of soap when drinking/eating. What does scientific research say?" If you are saying that minor traces of soap will not remain, that's an exceptional claim that needs an actual reference.
    – Dan Getz
    Commented Mar 4, 2015 at 14:07
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    "I wonder" is a valid question, but explaining the science for saponifacation isn't? "Soap is a surfacant(reference linked) and attaches to water" if there is no water on the plate, then there is no soap either. No soap = healthy. This is junior high science. The question is asked to justify an "ew" factor.
    – MentalMonarch
    Commented Mar 4, 2015 at 14:13
  • Replace "I wonder if it is" with "Is it", and it's a more straightforward question. How small an amount do you consider to be "none"? Does being a surfacant mean that soap will evaporate along with the water?
    – Dan Getz
    Commented Mar 4, 2015 at 14:18
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    It's indeed not a direct answer to the question, or the formulation of both the question and answer could be improved upon, but i consider this enough information for it to be -somewhat- of an answer. If anyone feels like pushing this question further, a separate question about the toxicity of common dish-washing-detergents could be asked, as MentalMonarch pointed out.
    – Wouter
    Commented Mar 4, 2015 at 14:59
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    This does answer the question, though it could be clearer. It seems to say "not rinsing is not unhealthy because the soap will slide off, so you won't eat it". I don't know if that's correct, but it's definitely an answer. I think an edit and a citation for the conclusion would make this a sterling answer.
    – Esoteric Screen Name
    Commented Mar 5, 2015 at 0:46

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