Clayton, N.C. — Johnson County opened an evacuation shelter for a second night Wednesday for residents who choose to stay out of homes in the area where a smoky fire at a yard-waste dump continued to make breathing uncomfortable and, perhaps, risky.

The smell of the smoke was noticed in Raleigh. City fire officials said there were no fires in the city to account for it. It was evident inside the newsroom at WRAL. At nearby North Carolina State University, campus police said smoke alarms were going off in some buildings and no other cause was evident.

Officials had decided at midnight Tuesday to use the reverse 911 phone notification system to tell approximately 6,000 residents in a three-mile radius of the dump on Loop Road about the shelter at Clayton High School. The decision came after air monitoring found unacceptable levels of carbon monoxide.

The precautionary shelter re-opened at 7 p.m. Wednesday, the county said. No one had arrived by 10 p.m. Three families stayed there overnight Tuesday, they said.

The warning targeted people with respiratory problems that could be made worse by the smoke, but officials continued to suggest that everyone living near the fire stay indoors whenever possible, keep windows and doors closed, and not use heating or cooling systems so they would not draw air into houses.

WRAL Chief Meteorologist Greg Fishel said a weather condition called a temperature inversion, along with no wind, meant it was likely the smoke would hang over the fire area until Thursday morning, when increasing winds were expected to help disperse the pollutants.

The Johnston County fire is a reminder of a spontaneous combustion fire that broke out at the city of Durham's yard-waste dump last September and burned for two weeks. That fire, which also poured smoke copiously, went on until firefighters brought in a chemical fire suppressant. That fire broke out Sept. 10 and lasted until Sept. 24.

Firefighters continue spraying water Wednesday as the smoke continued to pour from the ground where buries stumps and other tree debris caught fire Sunday. At times nearby homes disappear in a thick cloud.

"I have problems anyway breathing, and I've had a headache for the last two days I normally don't have," said Wilford Gay, who watches the fire from his back porch.

Firefighters knocked on his door late Tuesday and suggested he move out because carbon monoxide readings in his house were high. Gay opted to stay with his dog and other animals, however.

Johnston County Fire Marshal Harold Henrich said several crews will test levels. Anything above normal could result in mandatory evacuation orders.

“They should take it very seriously as far as the smoke, the carbon monoxide,” Henrich said.

State and federal environmental regulators have time-exposure standards for carbon monoxide. One standard involves the amount in outdoor air for an eight-hour period. The other, higher level is for a one-hour period. Neither government has official standards for indoor air.

The fire is at Stump Dump Inc. Officials said the company covered its yard waste with dirt, which is standard practice. However, the fire started under the dirt through spontaneous combustion.

The fire is expected to burn for several days, causing heavy smoke throughout the day and night. Loop Road has been shut down, except for residents and emergency traffic.

Johnston County emergency management officials said people with immediate concerns could contact them at 919-989-5050.