Since the Nappy Science Gang project started, our volunteers have asked hundred of questions in our Facebook group. We have also invited a number of experts from all sorts of different disciplines to have live online chats with us and enlighten us with some of their knowledge. And this is the result: a document where some of our most pressing questions are met with an answer from one of the guest experts. We hope you will find this useful!

Q: Is there a difference between liquid or powder detergent, in effectiveness and in build-up?
A: It all depends on the chemistry of the respective products. Powders are generally more basic, dissolve better and rinse out better compared to liquids. Liquids tend to have more chemicals in, because of the need for emulsifiers. This doesn’t mean they are definitely less effective, but they can give different results. There is no scientific evidence showing that liquid detergents “coat” fibres therefore causing more build-up. (Ian Strudwick)

A: We have definitely found powder detergents to be better at stain removal in our tests (Verity Mann)

Q: Is it best to use bio or non-bio?
A: In terms of detergent, bio detergents give better stain removal because of the way they react with the stain by ‘eating it away’. However, these can lead to detergent build-up which can give skin rash issues. Non-bio detergents react a different way, using chemistry in association with the heat, moisture and agitation in washing to release the stain into the liquor and hold it there. (Ian Strudwick)

A: If we look in a formulation strategy for stain removal, in general, powders are better than liquids, products for ‘whites’ are better than products for ‘colours’, biological detergents are better than non-biological. So a white biological powder has the greatest stain removing potential (Mark Smith)

A: I can only really speak from the studies that I performed on the five surfactants I used (3 synthetic, 2 bio), but I found that the bio ones actually had better wetting properties and better foaming than the synthetic ones (Jessica Liley)

A: Biological detergents work better on protein based stains as obviously they contain enzymes. The also contain bleaches and optical brighteners so whites look whiter. (Trisha Shofield)

Q: Is pre-rinsing in the washing machine a good idea? And is it best to do it with warm or cold water?
A: Pre-rinsing may also help with the mechanical action of the washing as the water soaked nappies are heavier to start with (Jessica Liley)

A: Since poo is a protein-based stain, we suggest to keep the wash temperature low-ish. So ideally, you would rinse first in cold water and then do a low wash cycle in a machine (Trisha Schofield)

Q: Is the use of a sanitiser recommended?
A: The general consensus between libraries and nappy retailers is that not only they are not needed, but that some of them (e.g. Dettol) are a bad idea from an environmental perspective. As with adding any antimicrobial to a wash on a regular basis, the risk is that you’re going to build resistance (Penny Broderick)

A: Laundry sanitisers have a slight benefit but at low temperatures it is negligible. Use of these (and the whole 30 deg C washing issue) is really just marketing by the manufacturers (Ian Strudwick)

Q: What effect does temperature have on detergent action? Is there much of a difference between washing at 30 and washing at 60?
A: From a scientific point of view reactions happen faster and more easily at higher temperatures, and also there is a linear decrease in surface tension of water as temperature increases, meaning if the water is hotter then it can wet things more easily. Also, at 60 you’re more likely to kill bacteria (only “more likely” as the temperature rarely reaches as high as 60 and only for a short amount of time, and the detergent itself may kill some types of bacteria anyway) (Jessica Liley)

Q: Is there a temperature you can wash nappies at, without damaging them, that will kill germs and also thrush? Do we need to wash at 70deg C to kill thrush?
A: The temperature requirement for effective cleaning of an uncoloured cotton nappy is 60 degrees C, any lower and the cleaning will be affected. Perhaps not noticeably, but at a microscopical level there will be a difference. If you want to sanitise the nappy by killing bacteria then you need to rise above 74deg C, but this gives other issues. Hospitals sanitise linens at 74deg C, but they also use different procedures, so you would never get a truly effective sanitisation using a domestic machine. By truly effective sanitisation I mean so clean that the nappy doesn’t support bacterial growth, but this is far more ‘clean’ than a nappy really has to be. So overall, washing at 60deg C with effective detergents is practically achievable and will give a perfect serviceable nappy for future use (Ian Strudwick)

Q: Is detergent build-up something we need to worry about? And if so, what should we do about it?
A: Detergent build-up certainly can happen, it’s not a myth. However it can easily be avoided with simple steps and it really comes down to common sense. Every time a nappy (or any other garment) is washed, it builds up a small amount of detergent that beds into the structure and reacts with the fibres. Over time this build-up can get so severe that it leads to odour formation and loss of absorption.

The solution is to give the nappy a treatment in excess of normal: on occasions, perhaps once a month, wash the nappies using a higher temperature than normal, say 60 deg C. As well as doing this, take steps to make sure that you are not repeating the detergent build up.

  • Firstly run the machine empty but with normal detergent, to clean out the pipes. Products such as Calgon can also help at this step.
  • Then do three rinses of the machine at the same high temperature but with no detergent, to rinse out the residual chemicals.
  • When you have a nicely clean, rinsed-out machine, wash the nappies at the same high temperature but without detergent. This loosens, dissolves and holds the built up detergent which is flushed away.
  • It’s probably best to wash the nappies three successive times without detergent just to make sure the cleaning process is effective.
  • Cleaning out the machine like this every three months is probably sensible and cost-effective for practical reasons.

(Ian Strudwick)

Q: How much detergent is the right amount of detergent?
A: During standardised cleaning tests run under tightly controlled conditions (using expensive washing machines, controlled water supply, fixed temperature and standard unsoiled loads) we have found that an effective clean is given if the sud height in the washing machine door window is 2-3cm above the water level. Any less and the cleaning may not be effective, any more and you are creating problems such as detergent build-up. Sud height is measured as soon as the wash load achieves its target temperature, which is anywhere from 10 to 30 minutes into the cycle depending on the temperature. However, if you run this experiment at home you might get different results since commercial washing machines vary enormously in their cycles and how much water they use, as well water hardness and detergent type all affecting sud formation (Ian Strudwick)

A: Another way of knowing if you are using too much detergent is to watch the last rinse cycle. If you are still seeing suds, you have used too much detergent and next time you should use a little less (Penny Broderick and Jessica Liley)

Q: what difference does water hardness make to washing nappies?
A: Soft water increases the propensity of chemicals to be dissolved into it. As a consequence, with soft water you may never have to strip-wash because the quality of your water means that the detergent is always effectively dissolved and removed in the rinse/spin stage (Ian Strudwick)

A: Hard water is mostly made up of positive magnesium and calcium ions dissolved in water, and these react with the negatively charged surfactants (you can get positive surfactants too but in laundry detergent you only have negative ones because they work against the charges of the clothes as discussed before). This reaction forms neutral compounds which precipitate out of solution and form scum. This really reduces the effectiveness of the detergent because they no longer have a charge, so that’s why in hard water environments foaming is more difficult, and cleaning is not as effective, whereas when you wash your hair in soft water, for example, an awful lot of foam is produced. Commercial laundry detergents contain builders which are basically water softeners, they complex with the hard water ions instead of the surfactant (Jessica Liley)

Q: Can you tumble dry bamboo?
A: Heat affects bamboo differently depending on how saturated the fabric is: when dry, it’s a lot more sensitive than when wet. You could lay wet bamboo boosters on a radiator at 75 degrees and they would be fine, but once they are dry they can’t tolerate it. If you leave a dry bamboo booster on a hot radiator, you’re likely to find the fibres falling out a couple of washes later (Penny Broderick)


Penny Broderick runs Worcestershire Nappy Advisory Service and is the founder of the Cloth Nappy Library Information Network. She has done a huge amount of nappy testing at home to try to investigate best washing routines, nappy absorbency and many more issues.

Mark Smith worked as water quality manager before moving to the Drinking Water Inspectorate where he was Research and Contracts manager as well as an Inspector. He then finished up being the Research Program manager for the then Department of the Environment, Transport and the Regions Water and Contaminated Land Research program and he now provides advice to various companies about issues related to water. This includes analysis, chemical, microbiological and biological, materials testing and emergency response.

Ian Strudwick is the technical manager of Shirley Technologies, a collection of laboratories involved in independent testing on textiles and polymers using standardised washing tests.

Trisha Schofield and Verity Mann are respectively Head and Deputy Head of Testing at the Good Housekeeping Institute. Both are responsible for testing large, domestic appliances such as washing machines and dishwashers and also a number of products and services.

Jessica Liley is a scientist who did her PhD on detergents and how to make them more environmentally friendly. She investigated the use of alternative “bio” detergents for use in laundry products, in the hope of ultimately replacing synthetic chemicals (surfactants) with biological ones (biosurfactants) derived from bacteria and yeast.