The one thing you must do to stop greasy hair

Watch: Viral hairbrush cleaning hack could explain why your hair gets so greasy

We know we should be washing our hair regularly to keep it clean, but turns out our clean-hair efforts are too often being scuppered by dirty hairbrushes.

A video showing just how dirty your hairbrush might be has gone viral, after clocking up more than three million views.

The clip, created by @everything_tidy, highlights just how much grime our brush bristles could be harbouring and the results could provide answers to some important hair-care questions, like why hair can get greasy even after it's been freshly washed.

The TikTok poster explained that she was inspired to wash her brushes after another creator (@unitfour) made a video explaining that hair can get greasy or oily quickly, not only because of hair follicles, but also due to the grubbiness of a hairbrush. Eugh!

She says: "So I’m gonna put that to the test. I’ve put all my brushes in the sink and we’re gonna leave them for a bit.

Read more: Can you train your hair to need less washing?

"I’ve used hot water and shampoo."

After putting her hairbrushes into the water to soak, she returns to the sink to inspect the water – only to find that it’s turned a dirty, brown colour.

"So I’ve left them for an hour and come back to check the water… oh my god, that’s disgusting," she continues.

"Ladies, don’t forget to wash your hairbrushes. I didn’t even know that was a thing, but look at the state of that."

It’s safe to say that people have been pretty grossed out by the results of the hairbrush-washing experiment.

"OMG I need to do this," one wrote.

"I'm doing mine today and I'm dreading it," another agreed.

Read more: Have we been washing our hair all wrong?

Others shared their own tips for hairbrush washing including rubbing the bristles together with another brush and using an old toothbrush for an even deeper clean.

Turns out there are quite a few of us who aren't adding 'scrubbing hairbrush' to our keep-it-clean lists.

We all know that regularly cleaning your make-up brushes is a must if you want to keep your skin bacteria free but somehow, our hairbrushes don't receive the same attention.

Watch: How to get rid of greasy hair — besides using dry shampoo

But if you've just washed and conditioned your hair and then pick up the hairbrush you haven't washed since you bought it, its bound to add some grime to your freshly washed locks, right?

According to a study conducted by the researchers at the University of Arizona, it turns out that hairbrushes are commonly the worst breeding ground for bacteria. In fact, they contain around 3500 colonies of bacteria per square inch.

According to Henry Paterson, senior operations executive at cleaning agency Housekeep, a grubby hairbrush could actually be riddled with dead skin cells and bacteria.

"Each time you brush your hair, you’re running oil, product build-up, dead skin and bacteria through the bristles," he told Drench.

"That also means with each sweep, they get added back into your hair – making it look greasy. Those who suffer from dandruff can exacerbate their problems by using a dirty brush."

Read more: This £8 tool will easily get the gunk out of blocked plugholes

For that reason, Paterson says it’s best to wash your hairbrush every month.

But, how do you even wash a hairbrush?

"Every time you use it, remove hair from the brush. Then, once a month shampoo the bristles to remove product build-up and dirt - you can use an old toothbrush here to scrub," Paterson says.

"Once, you’ve finished shampooing, rinse the brush in warm water. Make sure you don’t get the brush too wet as this can damage any wooden parts."

Watch: How often should you clean your hairbrush?