Health officials in Clark County, Washington, have declared a public health emergency for a measles outbreak in an area with a high rate of unvaccinated children.

As of Tuesday January 22, officials tallied 23 confirmed cases in the county, which is just north of Portland, Oregon. The case count rose rapidly, more than doubling since last week, and will likely continue to rise amid low vaccination levels in the area.

Nearly eight percent of children in Clark County were exempt from standard vaccination for the 2017-2018 school year, according to state records reported by The Washington Post. Breaking down that eight percent, about seven percent of kids had personal or religious exemptions, and the remaining one percent or so had medical exemptions. Factoring in the rest of the population, the county is below the 92-percent to 94-percent range some experts consider required to prevent the spread of highly contagious diseases such as measles.

In an outbreak report online, Clark County officials note that “measles is extremely contagious. The virus travels through the air and can stay up to two hours in the air of a room where a person with measles has been… Measles is so contagious that if one person has it, 90 percent of the people close to that person who are not immune will also become infected.”

Public health officials have assembled a lengthy list of locations in which people may have been exposed to the disease. It includes health care facilities, churches, schools, Concourse D and the Delta Sky Lounge of Portland International Airport, an Ikea, Dollar Tree stores, the Moda Center during a Trail Blazers’ game, and Amazon Lockers.

“It’s really awful and really tragic and totally preventable,” Peter J. Hotez told the Post. Hotez is a professor of pediatrics and dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston. “Portland is a total train wreck when it comes to vaccine rates,” he added.

Of the 23 confirmed cases so far, 20 were unimmunized. Eighteen of the cases were in children between one and 10 years old.

Correction: This article has been updated to correct the state in which Clark County resides. It is in Washington state, not Oregon.