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Several of my friends, when they do the dishes manually, do not rinse the dishes after cleaning but merely dip them in the dishwater and then dry them (using a towel or a dish rack). I myself on the other hand always rinse the dishes with fresh (hot) water from the tap before drying them, mostly because the idea of having a dose of soap with my next meal freaks me out.

But is there some guideline as to how much of the soap you can ingest until something goes wrong, be it the taste of the next meal or some poisonous reaction? The soap used for doing the dishes manually doesn't have any warning labels on it... I've found this describing poisonous effects of automatic dishwater soap, and this pertaining to a dish soap that is used for manual dishwashing.

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    $\begingroup$ I think you're missing a link in the deductive chain: you've gone from rinsing and drying versus drying, to amount of chemicals ingested; you've just assumed that drying without rinsing has as a consequence the ingestion of more detergent. (and there's a third and fourth way: rising, then air-drying; and just air-drying) $\endgroup$ – EnergyNumbers Dec 29 '13 at 10:15
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    $\begingroup$ While true, I think that the basic assumption "less rinsing: more soap" still holds. Even if you don't rinse and towel-dry, the towel will accumulate soap that may be deposited on the next item. $\endgroup$ – tschoppi Dec 29 '13 at 15:34
  • $\begingroup$ @tschoppi I tried that in the shower this morning. My towel needs washed and my skin wasn't smooth. $\endgroup$ – LordStryker Feb 26 '14 at 21:45
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I'm less familiar with safety regulations elsewhere in the world, but in the US, "chemical manufacturers" (which includes soap, sugar, salt, and just about everything else you can imagine) have to produce Materials Safety Data Sheets (MSDS) and make them publicly available.

Out of curiosity, I did some digging for soaps. Proctor and Gamble is a huge consumer producer, including dish detergent. So there are MSDS sheets for soap.

The MSDS gives oral LD50 values. That is, the lethal dose required to kill half the population.

The LD50 for soaps seem to vary by animal, but judging from a few MSDS I pulled up, they're ~2 g/kg or so. (That value was for rats.)

So if I had to guess, given, say a ~70 kg adult, we'd be talking about 140 g of soap, minimum to kill a person.

Now.. that's acute doses. I have no ideal if anyone has established long-term toxicity of soaps, and I can't find anything obvious online.

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    $\begingroup$ @tschoppi Actually, the biggest way to save water is to use an efficient dishwashing machine. I think the amounts of soap are likely to be negligible - consider it's a dilute wash solution and only a film will be left on the plate (if at all). I'd guess in the milligrams or micrograms of soap per plate. My preference for rinsing though is to make sure it goes through very hot water. $\endgroup$ – Geoff Hutchison Oct 8 '14 at 19:39
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    $\begingroup$ Well, I have to say that I don't have the funds necessary to finance a dishwashing machine. I am however aware of the high water efficiency that these machines exhibit nowadays. $\endgroup$ – tschoppi Oct 8 '14 at 20:03
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    $\begingroup$ One more thing to consider in addition to this fine answer is that the MSDS LD50 is for the pure active ingredient. Most products intended for contact with human skin are somewhere south of 20% SLS/SLES, so multiply the LD50 by about 5 to determine the raw volume of product required to be acutely dangerous. $\endgroup$ – KeithS Nov 24 '14 at 21:30
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    $\begingroup$ As far as long-term toxicity, the components of SLS/SLES are animal fats and various relatively common ions that your body deals with every day. The chronic long-term effects would lie largely in the denatured alcohol (translation; added methanol) common in most liquid detergent products, which over time will damage the nerves. $\endgroup$ – KeithS Nov 24 '14 at 21:33
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    $\begingroup$ @KeithS Agreed. Except in this case by the OP, the methanol is not likely to remain on the plate. I believe the poster wanted to know about soap residue remaining. After drying, the amount of methanol should be negligible. $\endgroup$ – Geoff Hutchison Nov 24 '14 at 21:37
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I am new here, however I have an Opinion. Soap from the dish water would not be the only concern for me. Food particles, oils,bacteria, etc., would be what would come to my mind. So, "for me" to leave soap ( or anything else on the dishes) negates the act of washing them in the first place. Just my opinion but I believe a thorough analysis of the rinsed and air dried dish as opposed to the un-rinsed and Toweled dish would show a significant difference in how "Clean" they were. In terms of Ingesting soap ( dish soap in this instance )being a health hazard I personally would consider ingesting ANY soap to be a health risk. I would think ( without being a Doctor) that such ingestion could Quite easily lead to Diarrhea, Irritable Bowels, the killing of essential enzymes and probiotics in the digestive tract just to mention a few things off the top of my head. There are probably many others but this Is what I thought of first. http://www.netwellness.org/question.cfm/19193.htm http://www.merckmanuals.com/pethealth/special_subjects/poisoning/household_hazards.html Detergents, Soaps, and Shampoos Exposures to products containing anionic and nonionic detergents generally cause mild gastrointestinal irritation that responds well to symptomatic care. These products include human and pet shampoos, liquid hand dishwashing soaps, bar bath soaps (except homemade soaps, which may contain lye), many laundry detergents, and many household all-purpose cleaners. Some of these, such as electric dishwasher detergents, are also alkaline corrosives (see Poisoning: Corrosives). All animals are susceptible. These agents are not well absorbed by the body, and toxicity is limited to irritation of the eye, mouth, or gastrointestinal tract, which is usually mild and resolves on its own. Vomiting and diarrhea are the most common signs. Dehydration and metabolic abnormalities can develop in rare cases after prolonged vomiting or diarrhea. Dilution with milk or water may reduce the risk of spontaneous vomiting. Vomiting usually resolves on its own after short periods of food and water restriction. In severe cases or in animals with sensitive stomachs, medications that prevent vomiting may be required. Rarely, fluid treatment is needed. Eyes should be flushed with water or saline for 5 minutes. www-ehow-com/facts_5802351_harmful-chemicals-dish-soap-html

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    $\begingroup$ The devil's in the dose. Those unpleasant symptoms seem to be referring to actually drinking the stuff. If this was the result every time someone left a few milligrammes of detergents on dishes, the OP's friends probably would have stopped doing it long ago. $\endgroup$ – Michael DM Dryden Oct 8 '14 at 22:27

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