Commercial pool operators are required by the state to add chemicals to the water to make it safe and pleasant for swimmers. Sometimes, those chemicals can be accidentally mixed with serious health consequences, like Monday night's incident at the Landmark Resort in Myrtle Beach that sent 18 people to the hospital.
The South Carolina Department of Health and Environmental Control regulates pH levels, or acidity in the water, in commercial pools.
"A balanced pH will provide eye comfort for swimmers," said Shannon Graves McKenzie, VP of Graves Pools & Spas of Myrtle Beach.
Often, that chemical additive is muriatic acid, but when it is accidentally combined with chlorine - which is also required to reduce bacteria in the water - McKenzie says it can cause a bad reaction.
"That causes ammonium chloride, which is a mist or a gas that can cause anywhere from minor mucus irritation to further damage to lungs or breathing, respiratory, something like that."
McKenzie says Graves Pools stopped supplying muriatic acid for its commercial customers a few years ago and went to a safer product.
Still, the acid they use now can cause problems if it's mixed with chlorine.
McKenzie added that her company stresses accident prevention, including proper pool training and certification for hotel employees.
Even then, she says, accidents can happen.
"If someone didn't look at a label properly, thinking it was the same product, they're both liquid, I can see where it can very easily happen," she said.
All Myrtle Beach firefighters are certified as hazardous materials technicians and their training targets the most common pool materials.
"Specific chemicals, chlorine, muriatic acid that we come in contact with on an annual basis and there are specific classes they go through just for those," said Asst. Chief Mike Norket.
In Monday night's accident at the Landmark Resort, Norket says fire officials worried at first that there were many more people hurt.
He says officials were happy with the way it turned out, with the chemicals quickly removed and the pool area cleared.