Apple Inc. announced this week it banned the use of benzene and n-hexane, potentially hazardous chemicals, in 22 of its final assembly factories, all of which are in China.

The decision came after a five-month campaign by Green America and China Labor Watch, which launched a petition calling on Apple to stop the use of the two chemicals in the production of iPhones and take responsibility for what they say are poor and inadequate conditions for workers in its supply factories. Around 23,000 people signed the petition concerned about the chemical exposure for workers.

However, a four-month investigation by Apple at 22 factories found no evidence that benzene and n-hexane endangered the roughly 500,000 people who work at the plants. No traces of the chemicals were detected at 18 of the factories, and the amounts found at the other four factories fell within acceptable safety levels, the company said.

The company, however, ordered its suppliers to stop using benzene and n-hexane, known carcinogens, during the final assembly of iPhones, iPads, iPods, Mac computers and accessories, and is requiring companies to test for the substances. The company is still allowing use of the two chemicals during the early production phases of its products.

"This is doing everything we can think of to do to crack down on chemical exposure and to be responsible to concerns," Lisa Jackson, Apple's vice president of environmental initiatives, said in an interview with The Associated Press. "We think it's really important that we show some leadership and really look toward the future by trying to use greener chemistries."

Apple made a $37 billion dollar profit in 2013. Its profit margin on iPhone manufacturing is 40 percent.

This announcement and the preceding investigation shows that Apple listens to its customers," said Elizabeth O'Connell, campaign director at Green America. "However, Apple needs to go further to create a safe environment at all factories in their supply chain for the health and safety of all 1.5 million workers."

Those 1.5 million workers are in 349 suppler factories that make Apple products. Advocates say workers don't receive training and lack protective gear when handling toxic substances in electronic manufacturing.

Green America said thousands of industrial chemicals used in the manufacturing of electronics -- some which are largely untested. And they are urging Apple to look more deeply into its supply chain, to the second and third tier suppliers, where chemical usage and safety procedures are less controlled.