SLO’s water makes her ‘itchy from head to toe,’ woman claims. What’s going on?
Two months ago, Caroline Bennet began to experience severe skin irritation after showering — so bad, that she’d scratch herself in her sleep until she bled.
In her almost three years living in San Luis Obispo, Bennet had previously noted some milder itchiness twice before, but it went away after about a week.
This most recent bout, however, felt different.
“After I shower, I’m just itchy from head to toe,” she said. “It just feels like ants crawling over my skin.”
Bennet believes her skin irritation is caused by trihalomethane, a byproduct of chlorine and organic matter interacting in the water.
The city adds a small amount of chlorine to the water to prevent it from growing microorganisms while traveling through the pipes. When chlorine interacts with organic matter, like leaves, it creates trihalomethane.
The city’s drinking water contains trihalomethane levels lower than the federal limit — so it is safe to drink and shower with, San Luis Obispo Public Works and Utilities Director Aaron Floyd told The Tribune.
But Bennet says a water sample taken from her house is higher than the federal limit.
Floyd, however, said a home sample doesn’t reflect the state of the city’s water quality as a whole — it only shows the quality of the water in that house on the day the sample was taken.
“The city’s water is safe for drinking water and all purposes,” Floyd said.
Floyd acknowledged that residents with health conditions or sensitivities could have different reactions to the water, even if it meets safety standards.
The Tribune looked into the issue as part of its Reality Check series.
City water meets state and federal safety regulations
To get to the bottom of her symptoms, Bennet sent a water sample from her home to a private Fresno lab called BSK Associates on Feb. 5.
On Feb. 20, she received the test result, which revealed that her water contained a trihalomethane concentration of 83 parts per billion — a slightly higher level than the limit of 80 parts per billion set by the U.S. Environmental Protection Agency.
Long-term exposure to higher levels of the pollutant are linked to increased risk of cancer and liver, kidney or central nervous system problems, the agency said.
Bennet blames her irritated skin on the high level of trihalomethane in her water.
However, Floyd said the sample from Bennet’s home does not accurately reflect the quality of the city’s drinking water supply as a whole.
Water quality in a house can be influenced by plumbing conditions, the amount of time it spends in a water heater, how long it sits stagnant, and more — all which can impact the concentration of trihalomethane there, Floyd said.
Despite the result of Bennet’s test, the city’s drinking water complies with federal regulations.
Drinking water providers must collect four water samples every three months and test them for trihalomethanes, according to Blair Robertson, a spokesperson for the California State Water Resources Control Board’s Division of Drinking Water, the agency that enforces federal drinking water regulations locally.
Those samples must be collected in February, May, August and November. Then, the provider takes an average of those samples to determine the level of trihalomethane in the water.
In San Luis Obispo, the current average for the four sample locations is 55 parts per billion, 25 parts per billion, 73 parts per billion and 31 parts per billion — all lower than the federal limit for trihalomethane, Robertson said.
Taking the average helps the city understand variations in water chemistry caused by fluctuations in water usage, source water variability and temperature, Floyd said.
If Floyd could shout any message from the rooftops, it would be this: San Luis Obispo’s drinking water is safe to drink, cook with and shower with.
However, he acknowledged that people with different sensitivities or health conditions could be irritated by the water, even if the water complies with state and federal regulations.
“While the city takes all concerns seriously and empathize with your described physical conditions, we continue to advise that if you are personally sensitive to disinfectant byproducts, the most practical and effective solution is typically installation of a home-level treatment system, such as a reverse osmosis system,” he wrote in an email to Bennet, which he shared with The Tribune.
Do trihalomethane concentrations fluctuate?
Trihalomethane levels in the water can change based on the amount of chlorine used for disinfectant, the amount of organic matter present in the water and temperature, Floyd said.
The city supplies its drinking water from Whale Rock Reservoir, Lake Nacimiento and Santa Margarita Lake, also known as the Salinas Reservoir.
First, the water is sent to the city’s water treatment facility, where it's disinfected with ozone, Floyd said.
Then, each batch of water receives the same amount of chlorine before being sent across the city to customers. The chlorine prevents the water from growing microorganisms while it travels through the pipes, Floyd said.
If a batch of water contains more organic matter, it could produce more trihalomethanes when treated with chlorine, he said.
Based on her experience, Bennet said water from Santa Margarita Lake causes the most itching.
After she moved into her house about 2.5 years ago, she started experiencing skin irritation when she showered.
Twice during the past 2.5 years, she suffered through a week of persistent itchiness. Then, the most recent bout of skin irritation started in January and hasn’t ended, she said.
Each time her skin irritation flared, she called the utilities department — who almost always told her they were using water from Santa Margarita Lake.
That lake contains more organic matter, like leaves and algae, than Whale Rock Reservoir, she said. As a result, when it’s mixed with chlorine before traveling through the pipes, it develops more trihalomethane, she said.
When the city delivers water from Whale Rock Reservoir, or when Bennet leaves town, her skin feels better.
“It seems to be the water,” she said.
While different reservoirs have different levels of organic matter, Floyd said the city frequently flushes the water system to prevent a buildup of trihalomethanes. The city also mixes and aerates water tanks for this purpose, he said.
Still, Bennet is looking for relief.
She installed a small water filter for her shower to remove trihalomethane, but she’s still experiencing skin irritation, she said.
Besides, she said, water filters aren’t an accessible solution. Home-level water treatment systems can cost about $6,000, and there’s no certification to prove that it removes trihalomethane, she said. As a renter, it’s not realistic for her to purchase a water filter that services her whole house, she said.
“I don’t think that’s within reach of most people in SLO,” she said. “I don’t think they have $6,000 just sitting around.”
She urged the city to find a different method to disinfect the water, such as powder activated carbon.
“It would be better if everyone had access to water that doesn’t irritate their skin,” she said.
This story was originally published March 2, 2026 at 10:04 AM.
