The towels we dry ourselves with get a lot of use and pick up a lot of microbes along the way. But how long should you wait before throwing them into the laundry?

You have probably rubbed your body with one today already. But just how clean was that towel you dried yourself with? Many of us will pop them into the washing machine once a week, while one study of 100 people found about a third of them did so once a month. A few, according to one survey in the UK, admit to only doing it once a year.

And while those fluffy fibres might not show any signs of dirt, they are a breeding ground for millions of microbes. Studies have shown that towels can quickly become contaminated with bacteria commonly found on human skin, but also with those found in our guts.

Even after washing, our bodies are still covered in microbes and perhaps unsurprisingly when we dry ourselves off, some of these transfer onto our towel. But the microbes living on our towels come from other sources too – airborne fungi and bacteria can settle on towel fibres while they are hanging up. Some of the bacteria comes from the water we have used to launder the towels with in the first place.

In Japan, some households even reuse leftover bathwater for laundering the next day. One study by researchers at the University of Tokushima in Japan found, while this saves water, many of the bacteria found in the used bath water were then transferred to towels and clothing after being laundered.

And for those of us who prefer to leave our towels to dry in the same room as your lavatory, there is some rather disgusting news – every time you flush, you are likely giving any towels nearby a light dusting with bacteria from your toilet, along with specks of your family’s bodily waste.

Over time these microbes can start to form biofilms on towels that can even begin changing how our towels look. After two months, even with regular washing, the bacteria living on cotton towel fibres start to dull the appearance of the cloth. But perhaps unsurprisingly, the total amount of bacteria and the species of bacteria depends on the laundry habits in the household. The real question is, how worried should you be about the bacteria living on your towels?