As Americans stockpile disinfecting wipes and paper towels to clean their homes more often to reduce the risk of coronavirus, California’s state water regulators on Tuesday urged them to keep one thing in mind: Don’t flush them down the toilet.

Wipes and paper towels do not break down like toilet paper does in water. They are stronger, and many wipes include plastics and materials like nylon. That means bad news for sewer systems, some of which already are experiencing problems during the coronavirus crisis.

“Flushing wipes, paper towels and similar products down toilets will clog sewers and cause backups and overflows at wastewater treatment facilities, creating an additional public health risk in the midst of the coronavirus pandemic,” the California’s State Water Resources Control Board said. “Even wipes labeled “flushable” will clog pipes and interfere with sewage collection and treatment throughout the state.”

They should be thrown away in the trash after use, the agency said.

The wipes, which kill most bacteria and viruses, are in huge demand due to the spread of COVID-19.

“In normal times when folks aren’t at home all day long, these wipes cause problems,” said Jessica Gaugher, legislative director for the California Association of Sanitary Agencies. “But now that we have people at home all day long, we are preparing ourselves for what might be coming.”

The wipes can wrap around tree roots and broken joints in sewer laterals between people’s homes and the street, causing toilets to back up. They can tangle in motors at wastewater plants and pump stations, and cause sewage spills.

In recent days, two Marin County wastewater agencies had their systems clog and spill due to an increase in wipes and other debris. The agencies, Las Gallinas Valley Sanitary District in San Rafael and Marin Sanitary District #5 in Tiburon, normally have very few spills, but an increase in wipes and other debris were to blame.

The Las Gallinas system spilled 550 gallons of sewage onto the street in a business district of North San Rafael on Tuesday when wipes and paper towels caused a clog in the sewer main under the road. Crews cleared the blockage in 11 minutes, but the sewage flowed through a manhole cover, across the road, into a nearby storm drain, where it was diluted with rain water, and flowed toward the San Francisco Bay.

“This is considered a small spill but I sincerely hope it is not a harbinger of things to come,” said Mike Prinz, general manager of the Las Gallinas Sanitary District. “I’m sure it’s already happening in a lot of locations all over the country.”

Apart from increased cleaning, one thing that may be happening, experts say, is that people who have run out of toilet paper are using paper towels and other materials in the bathroom instead. Prinz said if that is case, they should put the used paper in a plastic bag, seal it, and dispose of it in the trash rather than flushing it.

“The pipe you block may be your own,” he said. “You can block your own lateral to the street. Then your toilet won’t flush. Your shower won’t drain.”

Other Bay Area agencies are seeing the trend.

“Our staff is seeing higher accumulations of wipes, shop towels, that kind of thing,” said Steve Moore, general manager of the Ross Valley Sanitation District, which serves 47,000 people in Larkspur. “We’re seeing a notable uptick. We have to get the pitch forks out more often to clean the screens at the pump stations. We have to do that to keep the pumps working.”

Even before the coronavirus crisis, wipes already were costing California cities and other government agencies at least $50 million a year to untangle pumps, clear blocked sewer mains and increase maintenance to remove them at wastewater plants, she added, and the risk is high now that the problem could worsen.

A bill pending in the California Legislature, AB 1672 by Richard Bloom, D-Santa Monica, would require the makers of wipes to put a label on packages saying they should not be flushed.

New York City spends $20 million a year breaking up large clogs in sewer lines that are often caused by wipes. Workers in London have removed enormous blockages of grease, wipes and other debris nicknamed “fatbergs” — some of which are as big as city buses — from that city’s underground Victorian-era sewage pipes. American sewer plant workers also call such clogs “turkeys.”

Bay Area cities have struggled as well. Officials who oversee wastewater treatment plants say the last thing needed now is to have sewer systems face such problems.

“As people are sheltering in place, if we don’t get them to understand that we don’t want them to flush those materials, we could see an increase” in sewer system problems, said Kerrie Romanow, director of environmental services for the city of San Jose. “We want people to keep surfaces clean and to disinfect, but we want them to put paper products into the trash can.”

Some sewage plants filter huge amounts of waste every day. The San Jose-Santa Clara Regional Wastewater Facility in Alviso treats an average of 110 million gallons of sewage and wastewater from toilets, showers, drains and sinks a day — enough to fill 166 Olympic swimming pools every 24 hours. The plant serves 1.5 million people across Silicon Valley before treating the water to advanced levels and disposing it into San Francisco Bay. Some is recycled for irrigation.

The Centers for Disease Control and Prevention has recommended that Americans disinfect counter tops, doorknobs, tables, TV remote controls, light switches and other hard surfaces regularly to reduce the spread of coronavirus.

“Even during a time of crisis like this, we should be sure we’re not doing things that might inadvertently cause sewer line blockages and sewage spills that could harm public health or San Francisco Bay,” said Sejal Choksi-Chugh, executive director of San Francisco Baykeeper, an environmental group based in Oakland.