A massive haul of counterfeit makeup seized in Los Angeles turned out to be super-gross: testing revealed it contained bacteria and human and animal faeces, which unsuspecting buyers had been rubbing into their faces.

Detectives from Los Angeles Police Department's (LAPD) counterfeit and piracy unit together with the FBI started an investigation into the counterfeit products after customers called the actual companies complaining about bumps and rashes - noting they had all been purchased in Santee Alley in the LA fashion district.

Some of the brands being counterfeited included highly popular ones like Urban Decay, MAC, NARS, and Kylie Cosmetics by Kylie Jenner. Police seized a haul worth US$700,000 from 21 stores.

On Twitter, police captain Marc Reina announced the bust, highlighting contamination with bacteria and human waste.

"The best price is not always the best deal!" he said.

Legitimate companies tend to adhere to health and safety standards, but counterfeit operations cut corners.

Contaminants are rampant - and the products tested by the task force "were positive for elevated levels of bacteria, lead, and traces of animal feces," Reina said in a press conference.

"Those faeces will just basically somehow get mixed into the product they're manufacturing in their garage or in their bathroom - wherever they're manufacturing this stuff," detective Rick Ishitani told KABC.

Rubbing faecal bacteria on your face isn't just super gross, it's also dangerous. Since research in the 1970s led to the introduction of preservatives in cosmetics, bacterial infection is much more rare than it used to be, but the risk is much higher with unregulated counterfeits.

With eye makeup in particular, there's a risk of chronic eye infections and corneal damage.

Of the 21 businesses the task force raided, six owners were arrested, with the others issued cease-and-desist orders.

Ishitani said that it's relatively easy to tell which cosmetics are counterfeit - the price tag is a dead giveaway.

"If you're getting something that's 50 percent off, 75 percent off - it tells you that it's bad," he said.