Robotaxi company Zoox, owned by Amazon, has grounded its test fleet in Atlanta after safety drivers may have been exposed during August to carbon monoxide, carbon dioxide or hydrogen sulphide inside the vehicles themselves. One of them filed a complaint with OSHA, the US Occupational Safety and Health Administration, which opened an inquiry and ordered the company to investigate the cases.

OSHA's letter to the company states that up to 40 workers may have been exposed. Zoox says the number is lower: it identified 28 employees who rode in six vehicles with an "unexplained odour", of whom 10 "voluntarily" went for a medical check. It received, it says, only "one medical report establishing CO2 exposure".

The worst day was 14 August. Four workers were riding together in one SUV - a practice Zoox uses when training new hires. "Everything was fine, we were laughing and talking, and then suddenly everyone went quiet," says Jay Davis, hired through the staffing firm HireArt. At the morning break, as soon as they got out of the vehicle, one colleague started vomiting and the others felt dizzy. The vehicle was towed to the base and the workers were sent back by Uber. Then: "We thought we'd get lighter tasks for the day. No, they put us in another vehicle and told us to get back to work."

After a whole afternoon of driving, Davis started throwing up after a few bites of food. Three of the group ended up in the emergency room, where they were told their symptoms were consistent with carbon monoxide poisoning, although in Davis's case the doctor found no elevated levels. Workers kept falling ill until 22 August, when someone at the depot called an ambulance for a sick colleague. The company's response to the four who complained first: an order for what it called a "random" drug test.

What exactly was in the air still isn't clear. Zoox says it found no carbon monoxide, which is odourless, and that the only gas detected in the vehicles was CO2. It replaced the 12-volt batteries - damaged batteries can release hydrogen sulphide, which smells of rotten eggs, the smell some drivers say they noticed - but later said it found no damage to the batteries. The company promises gas monitors in the vehicles, and OSHA has since closed its inquiry.

There's also the detail that sets Zoox apart from its competitors. While Waymo, Tesla, Avride and Motional test with electric vehicles that have no exhaust, Zoox uses modified Toyota Highlander SUVs with internal combustion engines for its test fleet - and keeps them running most of the day so the sensors don't lose calibration. Its actual robotaxi vehicles are fully electric and are not affected. The US road safety agency spent six years investigating Ford over carbon monoxide in the Explorer, and found the highest levels precisely in police vehicles that had been fitted with sirens, lights and other equipment. Every modification can increase the risk of gases getting into the cabin.

Zoox is racing to catch up with Waymo, expanding into new cities and scaling up operations in Las Vegas and San Francisco. The company says the "safety and well-being" of its operators are its "primary concern". Back in August, Davis wrote to HR that he considered it wrong that they had been sent back out. When someone says they feel sick and the answer is another vehicle and a drug test - whose safety is really the priority?