The Temperature Question: Which washing temperature is really best? (Part 2)


Following on from Part 1 of our investigation into how washing at different temperatures affects the lifetime of a nappy, this part of the experiment looks at how clean nappies get when washed at different temperatures (as well as few extra questions besides!) so we can get to the bottom of which washing temperature is really best.

Hannah KinsellaAmy Fowler

Photos taken by participants during their experiments

The experiment: what we did.

The plan for this part of the experiment was to gather a ‘poo soup’ together from babies poo sent in by volunteers and then run some lab tests to see how clean nappies get when washed at different temperatures.

So first off, volunteers sent in samples of their kid’s poo (which in case anyone was wondering, you can send by Royal Mail with a bit of extra packaging!) which were cultured, strains of bacteria extracted and then mixed up into a delightful ‘poo soup’. Then, or wonderful lab guru Dani got to work using the poo soup to dirty nappies, then washing them, swabbing them and finally working out how many bacteria were left. She tried all the same temperatures as the first part of the experiment (that’s 30, 40, 60 and 90 degrees) for different materials of different thicknesses and different ages. Old nappies battered nappies from a nappy library were used to represent the old nappies, while brand spanking new ones used for the new ones. Four layers of fabric were used to see if layering fabric caused bacteria to be retained, something like what goes on in a booster pad.


The results: What we found.



To start, let’s have a look at how clean the different fabrics got at different temperatures. From the graph below, we can see there is a trend towards lower the bacteria levels when the materials are washed at higher temperatures (especially 90 degrees C) which is what we’d expect if more and more bacteria were killed off by the heat.

However for the PUL waterproof fabric, the level of bacteria goes up when it’s been washed at 90 degrees. If we look back at part one of this experiment, it seemed that some of the nappies washed at 90 degrees showed damage to the PUL coating at this temperature, which could mean that it is easier for bacteria to accumulate, though since bacteria usually grow at a similar rate on different fabrics we’d have to look into this further to be sure.

We can also see that the four layers of bamboo, representing a bamboo booster, remains the dirtiest at all temperatures (perhaps some more detailed experiments are needed to see if we think bamboo is as naturally antibacterial as it is touted to be!) , while wool, fleece and PUL waterpoofing were the cleanest. We can also see that the four layers of fabric were usually less clean than the single layer.


temperature vs bac score2

Graph showing how clean different fabrics after being dirtied with the ‘poo soup’ and washed at different temperatures. X4 indicates four layers of fabric. Units: CFU/cm3 are the number of colony forming units per unit area of the agar plates the colonies were grown on.

It also seems that for some of the materials they were cleaner when washed at 30 than at 40 degrees, while one material (the microfiber) was cleaner at 60 degrees than at 40 degrees. This was not what we were expecting as we thought the hotter the temperature the more bacteria would be killed. Since the bacteria levels between 30, 40 and 60 are not hugely different, another experiment with more trials would be needed to confirm this.

Now, if we look at old vs new fabric, we can see again 90 degrees definitely gets the nappies cleanest, and that as we might expect, older nappies do not get as clean as brand new ones. This could be because the old ones harbour bacteria colonies, or it could be because the composition of the fabric has changed with washing (check out part one of the blog for some more ideas on this!). Either way, it would certainly be an interesting follow up experiment to try to mark the nappies each time they were tested to see if they were harbouring a constant colony.


temerature overall
Graph showing the bacteria levels for old and new nappies washed at different temperatures. Units: CFU/cm3 are the number of colony forming units per unit area of the agar plates the colonies were grown on.

So, in summary:

  • 60 and 90 degrees usually got the fabrics cleaner than 30 and 40 degrees
  • Four layers of bamboo was the dirtiest fabric tested, while PUL waterproof material, fleece and wool got the cleanest
  • Older nappies were dirtier than new ones, whatever temperature they were washed at.

Looking back at parts one and two of this experiment, which temperature really was best? Well, washing at 90 degrees did get the nappies the cleanest, though with some expense to the nappies lifetime including shrinkage and damage such as balding and loss of the waterproof coating. It also might mean that the PUL waterproof material can be home to more bacteria than it would otherwise. 30, 40 and 60 had similar cleaning effects, while the hotter temperatures did progressively more damage to the nappies.

There’s also plenty of room for more experiments working out more clearly what damage hotter temperatures did to the nappies, for example the elasticity or absorbency, and also deciding if 30 degrees really does get nappies cleaner that 40 degrees (as our results above seem to show!), but for now let’s be content that we, the Nappy Science Gang, have crossed new frontiers in citizen science!

2 thoughts on “The Temperature Question: Which washing temperature is really best? (Part 2)

  1. […] The gang of cloth nappy users wanted find scientific answers to questions about cloth nappies, but had found that the science that was out there didn’t ask any of the questions that were relevant to them. So, the Nappy Science Gang set out to answer them – beginning with what washing agent is really best for cloth nappies. After many weeks of discussion about the experimental protocol, the group designed and carried out two experiments- the first to see how washing at different temperatures affected the lifetime of a nappy, and the second to see how clean the nappies got when washed at different temperatures. This blog is about the first part of this experiment, and the second is described here. […]


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s