Guests at a hotel in Sydney, Australia had to be taken to hospital after a staff member cleaning the pool mistakenly mixed pool chlorine with hydrochloric acid - creating a cloud of poisonous chlorine gas that seeped through the hotel building.
According to the Australian Broadcasting Corporation (ABC), 30 guests were treated on location at the Pullman Sydney Hyde Park Hotel for respiratory problems and eye irritation, and 8 were taken to hospital for further treatment.
Guests at the hotel started complaining about a smell at about 9 am on Wednesday 19 September. Firefighters called to the scene detected fumes on the 22nd floor, where a pool is located.
"The fumes were naturally emanating from the container and they got into the air-handling system and the lift shafts," NSW Fire and Rescue duty commander Charles Begley told the ABC.
"If you're exposed directly to them long term, it can be quite serious for your respiratory system."
Chlorine and hydrochloric acid are both used for pool maintenance. Chlorine, usually in solid tablet or liquid form, is a strong antibacterial agent used to treat the water to keep it clean and germ- and algae-free.
Hydrochloric acid, on the other hand, has a number of uses, including cleaning pool tiles and - when used very, very carefully! - lowering the pH of pool water.
Each chemical on its own is mostly harmless if handled properly, but when mixed, they generate chlorine gas. This is the chemical's natural state, and it's highly poisonous.
"Chlorine gas is extremely irritating to eyes and lungs. It was used notoriously in World War I and more recently as a chemical weapon in the Middle East," said chemist Ian Rae of the University of Melbourne.
In high concentrations, chlorine gas can be deadly. But even low exposure can cause problems, such as difficulty breathing, burning sensations in the airways, shortness of breath, nausea and headaches.
Most people affected recover within a few days, but there is a chance for chlorine gas exposure to result in long-term respiratory problems; when inhaled, the gas can combine with water in your tissues to form hydrochloric and hypochlorous acids, which could cause chemical burns.
For safety reasons, experts often recommend that hydrochloric acid and pool chlorine should never be stored together. They should absolutely never, ever be mixed, under any circumstances.
Guests at the hotel said that the smell was intense. People were coughing, and their eyes were stinging. Firefighters evacuated the hotel's 20th, 21st and 22nd floors.
"It has a choking smell and it produces rapid symptoms of irritation, so exposed people are likely to fear the worst, even if they have been exposed to just a little of the gas," Rae said.
"The effects depend on how much of the gas people have been exposed. It's a scary experience, and medical attention is urgently needed."
According to the Sydney Morning Herald, all the patients are stable and doing well. Firefighters had finished ventilating the floors by 11 am, and the hotel was able to resume operations.
Still, this unfortunate mishap is a great reminder to pay attention to the instructions on your household supplies, and make sure you're aware of the dangers of the simple, yet dangerous chemicals we all can so easily access.