[chris (OP)]

Caitlin says:

I live in a studio apartment without an automatic dish washer and the topic of detergent consumption often comes up with my boyfriend after I've handwashed the dishes.

The hypothesis is that washing dishes manually leaves some detergent residue which we consume next time the dishes are used for eating/drinking and this residue is harmful.

Furthermore, automatic dishwashers are safe because the detergent residue levels have been tested and are controlled.

We are both scientists and his argument seems reasonable but I have been unable to find any solid evidence one way or the other.

Are the residues from common manual dish washing (not containing antibacterial additives) significantly toxic to humans in the short or long term? 

Thank you kindly for your answer in advance!

What does everyone think?

[alancalverd]

Dishwashing detergents are scrupulously tested for ingested toxicity but it is quite possible that some people may have an allergic reaction to almost anything - though this is usually expressed as dermatitis.

However apart from the waste of time, water and detergent associated with hand dishwashing, the process is too inefficient to remove all residues and the water temperature is too low to kill any bacteria that remain in the grease film. Glazed crockery should feel squeaky-, not slippery-clean.  Precise pre-treatment of the wash and rinse water, combined with the high temperatures and optimised detergent, make automatic dishwashing the better choice for every reason.   

It has been argued that antibacterial additives increase the longterm buildup of resistant bacteria, either on the crockery and work surfaces or in the gut.

[evan_au]

Detergent is designed to be highly water-soluble.
If you are concerned, rinse the dishes in clean water before drying.

[alancalverd]

Problem is that detergents have a hydrophilic and a lyophilic end to their molecules.  If you have some well-bound grease on a surface, it is quite possible for the lyophilic head to stick to the grease but at a low water temperature and with insufficient abrasion the hydrophilic tails may not have enough energy to form dispersant micelles  and you just end up with a monolayer of surfactant stuck to the grease. Hence the "slippery clean" test.

Handwashing dishes probably won't kill you, but it will waste your life - and a lot of water

[nutritionalscientist]

Emulsifiers used in foods (even those extracted from eggs and soy) are basically surfactants or detergents.  The latest research seems to indicate that these emulsifiers in foods and remnants of dishwashing detergents and rinse aids in dishwashers are able to damage the protective barrier in the intestines and therefore allow bacteria to infiltrate the cells causing inflammation, instead of being kept well out of harms way. When it comes to washing dishes - make sure they are rinsed well in plain water - I've heard white vinegar makes a good alternative rinse aid in dishwashers (though I haven't tried) as for foods, I chose those without emulsifiers (shorter shelf-lives generally).

[chris (OP)]

We're going to cover this as a Question of the Week on the Naked Scientists programme, so your perspective is very valuable. Thanks.

[Bored chemist]

Emulsifiers used in foods (even those extracted from eggs and soy) are basically surfactants or detergents.  The latest research seems to indicate that these emulsifiers in foods and remnants of dishwashing detergents and rinse aids in dishwashers are able to damage the protective barrier in the intestines and therefore allow bacteria to infiltrate the cells causing inflammation, instead of being kept well out of harms way. When it comes to washing dishes - make sure they are rinsed well in plain water - I've heard white vinegar makes a good alternative rinse aid in dishwashers (though I haven't tried) as for foods, I chose those without emulsifiers (shorter shelf-lives generally).

Interesting.
Fats are broken down initially to fatty acids in the guts and those fatty acids are attractant/ emulsifiers.
One of the reasons that newborn pre-term babies struggle to breathe is that their lungs haven't developed the ability to make the surfactants they need.
In the normal way of things those surfactants are carried out of the lungs by the mucociliary escalator and swallowed.
So people always do, and always have  consumed surfactants.

It seems odd that "all surfactants are bad"
Any chance of seeing the actual research?

[syhprum]

Although hand washing of dishes may seem a time and water wasting chore in some ways it can be a relaxing and satisfying job, you start with a cluttered kitchen and end up with a pristine one the rinsing water at about 45°C will ensure  everything  dry's rapidly and you end up with clean hands and a feeling of a job well done. 

[cinka]

In my opinion, both has point but actually washing manually can be safe also and it depends on how you clean it after the final wash. You can wipe it with dry towel to get the residue if you are unsure.

[TimBandTech]

If you have some bacon grease and some wood ash you can make soap on your stovetop.
Soap is all natural. Yes, there is some chemistry there, and yes, the lye can be quite powerful, but it is also really very natural.

Maybe try making your boyfriend some Dove biscuits as a surprise.
Not sure actually if that would work but yeah, soap is pretty much a fertilizer.
No links are allowed here, otherwise I'd get you some proof.
Too bad this site is so locked down.

Caitlin says:

I live in a studio apartment without an automatic dish washer and the topic of detergent consumption often comes up with my boyfriend after I've handwashed the dishes.

The hypothesis is that washing dishes manually leaves some detergent residue which we consume next time the dishes are used for eating/drinking and this residue is harmful.

Furthermore, automatic dishwashers are safe because the detergent residue levels have been tested and are controlled.

We are both scientists and his argument seems reasonable but I have been unable to find any solid evidence one way or the other.

Are the residues from common manual dish washing (not containing antibacterial additives) significantly toxic to humans in the short or long term? 

Thank you kindly for your answer in advance!

What does everyone think?