Apple has banned the use of two potentially harmful chemicals in its final assembly processes after its own inspection turned up these substances in four plants assembling Apple products, Re/code reports.

The company disclosed its findings late Wednesday, at the conclusion of a four-month inspection of some 22 facilities responsible for assembling iPhones, iPads, iPods, Mac computers and accessories.

“We’re committed to removing toxins from our products and processes,” said Lisa Jackson, Apple’s vice president of environmental initiatives. “Because everyone has the right to a safe product and a safe working environment.”

Activist groups China Labor Watch and Green America launched a petition drive earlier this year, calling on Apple to abandon the use of benzene and n-hexane. These chemicals can also be found in substances including paint strippers, industrial cleaning products and household cleaners. Benzene is a known carcinogen, and n-hexane has been linked to nerve damage.

Apple investigated 22 final-assembly factories that employ nearly 500,000 people and found no trace of benzene or n-hexane in 18 of the facilities. In four facilities, it found evidence of the chemicals that fell within Apple’s previous safety limits, according to Re/code.

The company said it worked with the manufacturers to find safer alternatives and banned the use of the chemicals in the final assembly process. Apple will require its manufacturers to test all cleaning agents and degreasers before they’re used in production to make sure there are no hidden chemical risks.

Apple also said it would assemble an advisory board to further minimize toxins in the company’s products and supply chain.