A report into the treatment of workers making iPhones for Apple claims that 90 workers were taken to hospital in May last year after a leak of chemicals including sulphur and phosphorous at a Catcher Technology factory in China.

Rights group China Labour Watch (CLW) details the incident in a report, published on Tuesday in association with the Guardian, into the treatment of workers primarily making products for Apple, in factories in Suqian, Jiangsu province.

The report reveals workers’ claims that they are working long and gruelling hours for inadequate pay, as well as being exposed to chemical hazards. Among the claims are that overtime worked is nearly double the level allowed under Chinese law; that workers are paid a basic wage equivalent to £1.26 an hour; and that the plant discharges contaminated wastewater into the public drains.

The Catcher factories featured in the investigation produce items including the iPhone 8 metal frame and the keyboard case for MacBooks. Catcher also produces parts for other major technology brands, including Sony, IBM, Dell and HP.

Apple and Catcher both dispute the claims and say that workers were only taken to hospital after the gas leak as a precaution. A previous CLW investigation in 2014, which warned that workers were exposed to toxic chemicals, brought assurances from the tech companies that they expected suppliers to uphold the highest standards. But today Li Qiang, CLW’s executive director, accused the company of new failings.

“Apple has recently said they respect Chinese law but Apple has, in reality, been selective about which laws they follow, especially in regards to labour and environmental protection laws. Apple need to uphold their claim of honouring Chinese law,” he said.

And he added: “Catcher has been risking the health and safety of its workers as well as the local environment.”

The CLW report included payslip evidence to show overtime in excess of 70 hours in a month – nearly double the 36 hours a month normally permitted under Chinese law. Some companies are able to negotiate exemptions from the legal limit, and Apple insists the company has passed all of its audits and complies with overtime regulations.

The report claims workers were previously paid double for working on Saturdays but that new shift patterns meant that Saturday was now treated as a normal working day. It said workers were paid a base wage of 1,950 yuan (£220) a month for a nominal eight-hour day, for five days a week. But in practice, the report said, workers were required to work for 10 hours a day for six days a week and the additional day was not treated as overtime, meaning it did not have to be paid at the additional rate. CLW calculated that the change meant workers were losing about 500 yuan (£56) a month.

Apple and Catcher dispute the claim and insist workers are paid according to the law.

CLW’s undercover investigator described gruelling working conditions in one factory, where workers were mainly involved in producing the metal frame for the iPhone 8.

“We work without stopping, disassembling rigs, removing screws, assembling metal frames, and screwing in screws to fasten the rig. There is essentially no time to rest,” he wrote. He described standing up for hours on end on night shifts, working for hours without a break. “We would work from 1.20am to 5.50am, a total of four hours of work that is done standing up. There is practically no rest time. Our legs would fall asleep and our feet would hurt. The line leader would from time to time urge us, and shout: ‘Attach the screws faster.’

“When it was 3.30am, my entire body felt like my spirit had left me. My head, legs and feet were devoid of sensation and only my hands kept moving, mechanically disassembling rigs, assembling metal frames, fastening rigs.”

The report covered three factories owned by Catcher in Suqian. It claimed working conditions posed long-term health risks to workers and said the CLW investigator suffered ill health after working on a production line for a month.

“During the manufacturing process, chemical cutting fluids and metal particles would often splash into his eyes. Despite this, Catcher does not provide workers with safety goggles. While working at Catcher, the investigator developed symptoms of reduced vision, eye pain and irritation along with a persistent sensation of foreign substances in his eyes. One month after leaving the factory, the investigator’s eyes remained bloodshot.”

Concerns were also raised in the report about safety and access to the building, with workers expected to squeeze through a narrow opening one at a time. The report said that workers’ dormitories did not have fire exits.

An analysis of wastewater from the plant, included in the report, found that it exceeded local government standards for chemical oxygen demand, biochemical oxygen demand and levels of suspended solids – common tests for water quality. The sample was taken after the investigator spotted white foam discharging from the factory into the public drains.

The report claims five workers were treated in intensive care after last May’s gas leak, but Apple and Catcher dispute this. An interim report from the administrative committee of the Suzhou-Suqian Industrial Park blamed the leak on a mechanical failure. It said the gas permeated throughout the workshop, “triggering adverse reactions among operator personnel”. The committee said all affected staff were later released from hospital. Today, an Apple spokesperson said: “We’ve been working with the team at Catcher in Suqian, China, since 2012 and they’ve made significant progress raising standards during that time.

“Dozens of Apple employees are permanently on site, monitoring operations, and we’ve conducted 10 in-depth audits over the past five years, including three last year – and the last audit scored 96 out of 100.

“When we heard these latest claims we immediately sent a team of experts to the facilities. They thoroughly investigated and interviewed over 150 workers but found no evidence that Catcher was violating our standards. Our checks also confirmed wastewater is treated appropriately and protective equipment is provided to employees who need it, with detailed records maintained.

“We know our work is never done, and we investigate each and every allegation that’s made. We remain dedicated to doing all we can to protect the workers in our supply chain and make a positive impact on the environment.”

In a statement, HP said it “takes the working conditions in the factories that manufacture its products very seriously and has high standards in place with our suppliers. HP vigorously investigates all incidents and credible allegations.”

The other companies did not respond.