Image copyright Getty Images

Palm oil is used in everything from cosmetics to food to soap. There is enormous demand for it. But it is hugely controversial because it involves the clearing of tropical rainforests to grow palm oil plantations. This has led to the loss of biodiversity and habitat for under-threat animals such as the orang-utan.

But some companies are signing up to comply with a standard known as "sustainable" palm oil. BBC environment correspondent Claire Marshall explains.

There is a generally-agreed global standard set by the Round Table on Sustainable Palm Oil (RSPO), which is made up of oil palm growers, retailers, NGOs and manufacturers.

To become officially "sustainable", a company pledges not to clear any primary forest, to have transparent supply chains, to check how much carbon they are emitting, to limit planting on peatlands, to treat workers fairly, and create wildlife zones.

Who is using it?

In 2016 75% of the total palm oil imports to the UK were sustainable. Many large retailers and leading brands use it. Chester has declared itself the first sustainable palm oil city in the country.

Why haven't I heard of it before?

According to the largest player in the sustainable palm oil industry, Sime Darby Plantation, the companies that buy it often don't then make it clear on product labels. One reason could be that because palm oil is so controversial few companies want to be linked to it - even if it's sustainable.

Media playback is unsupported on your device Media caption Make-up artist Emmy Burbidge travels to Papua New Guinea to discover the truth about what’s in her products

What are the problems with it?

Several leading environment charities say the rules aren't strict enough and the sanctions aren't strong enough. A recent report by the International Union for the Conservation of Nature said there was "limited evidence of the conservation effectiveness."

So … why not just ban palm oil?

We are unlikely to stop demanding things like ice cream, soap or ready meals. The vast majority of these need vegetable oil to be made - and palm oil is the most productive vegetable oil crop on the planet.

Alternatives are sunflower oil, or rapeseed, or coconut. But palm oil is far more productive, yielding up to ten times more oil from the same amount of land. Put it another way: ten times more land would be needed to grow these other crops to get the same amount of oil.

Most leading environmental charities are not calling for a boycott, saying it would just shift problems to different parts of the world - and maybe make matters worse.

How can I find sustainable palm oil?

It's tricky, because often it's not used. But there is a label.

There is also a free app called Giki, recommended by WWF, where you use a smartphone to scan the barcode.

You can watch the full documentary, Unmasked: Make-Up's Big Secret on iPlayer. If you're from outside the UK you can watch the film here.