Regional air-quality officials are trying to find the source of a cancer-causing chemical discovered recently in Jurupa Valley at levels about four times higher than the average for urban Southern California.

“There is no cause for alarm, but we are looking into it because it is a bit unusual,” said Philip Fine, an assistant deputy executive officer for the South Coast Air Quality Management District.

The chemical is methylene chloride, a common industrial solvent used as a degreaser and paint stripper. It is very volatile, so it’s also used as an aerosol spray propellant and as a blowing agent for polyurethane foams.

Jean Ospital, the air district’s heath-effects officer, said the levels found in Jurupa Valley are too low to have short-term health effects.

But the long-term effect is a concern, Ospital said. The cancer risk is estimated at 7 cases per 1 million people over a 70-year lifetime, and the air district wants to eliminate that risk, he said.

“This risk is high for just one chemical, and it’s higher than the other communities in the air basin,” Ospital said.

In Jurupa Valley, the total cancer risk for all other sources of air pollution, including diesel soot, is 385 cases per 1 million people.

Fine said the air district first noted methylene chloride spikes in outdoor air samples taken at the district’s air monitoring station in the Rubidoux area of Jurupa Valley in late 2012 when collecting data for an air toxics study.

He said the district also found elevated levels in recent samples, as well as in samples collected separately by the state Air Resources Board, which monitors air quality for California’s Environmental Protection Agency.

The source, however, remains a mystery.

Air district officials have examined wind directions and other meteorological data, but they have been unable to trace the pollution to a residence or business, Fine said.

In 2004 and 2005, air quality officials found similar spikes in methylene chloride in Long Beach, and never were able to find the source.

Fine said the chemical is found in paint-stripping products readily available at retail stores such as Home Depot and Lowe’s. The air district, he said, may re-evaluate how it regulates such products, he said. It already has rules limiting the volatility of paints and varnishes because fumes contribute to the formation of smog.

Rachel Lopez, a Jurupa Valley resident and environmental activist, said she is glad the air district is trying to find and curtail the methylene chloride pollution.

“The air is bad enough here,” she said.

While regional air quality is improving, Jurupa Valley has pockets of people living near freeways, railroads and warehouses who are exposed to higher levels of pollution.

Penny Newman, executive director of Jurupa Valley-based Center for Community Action and Environmental Justice, said several industries in the area may be using industrial solvents.

“It is disturbing that they don’t know where it is coming from,” Newman said.

Methylene chloride isn’t the first unusual type of pollution in Jurupa Valley.

In 2008, the air district disclosed that it had found elevated levels of cancer-causing hexavalent chromium in the Rubidoux area.

District officials later traced the pollution to the TXI Riverside Cement plant, which was found to be releasing chromium-tainted cement dust into the community.

TXI was cited for dust violations and agreed to pay $1 million in penalties and to make changes to its operations in ways that greatly reduced dust emissions.

Contact the writer: 951-368-9471 or ddanelski@pe.com