One day after San Joaquin County officials warned residents to avoid smoke from the massive Medline warehouse fire, air-quality officials said the plume had caused no significant pollution at ground level — but a lung specialist cautioned that the readings do not eliminate every potential health risk.

County officials urged residents Thursday to stay away from the smoke and take precautions because materials burning inside the medical-supply warehouse could release harmful particles, gases and chemicals, one of which was described as a neurotoxin. By Friday, however, temporary monitors around the fire were reporting air quality in the good to moderate range.

None of the three monitors showed major effects from the fire’s smoke plume, said Jamie Holt from the Valley Air District.

“We were fortunate that it stayed aloft and began moving out of the area,” Holt said.

The fire began Thursday afternoon at the Medline Industries distribution facility in the 5700 block of Promontory Parkway. The warehouse became fully engulfed and sent a towering column of black smoke into the air that could be seen from Tracy, Stockton, and parts of San Joaquin County and those adjacent. Firefighters continued working at the site Friday as the building continued producing additional smoke.

Holt said the plume rose about 100 feet or more into the atmosphere before beginning to disperse. Officials had been concerned that changing weather conditions could bring the smoke back toward the ground overnight, but that did not occur.

“The smoke continued to go up into the atmosphere and not come back down to ground level,” Holt said.

Air quality around Tracy was in the federal Air Quality Index’s “good,” or green, category Friday morning before entering the “moderate,” or yellow, range later in the day.

Moderate air quality is not unusual in the San Joaquin Valley as temperatures rise and conditions worsen during the hottest portion of the day, Holt said. The district continued watching the readings to determine whether the warehouse fire was contributing to any changes.

The next category on the AQI scale is orange, which means air quality is considered unhealthy for sensitive groups. Those groups include young children, older adults and people with asthma, chronic obstructive pulmonary disease, cardiovascular disease or other medical conditions.

“If folks are seeing it reach that orange category, that’s when they really need to, especially if they’re a sensitive group, take action to make sure that they’re not being exposed to that poor air quality,” Holt said.

But the AQI isn’t the only scientific tool people should use to determine air safety.

Afif El-Hasan, a Southern California pediatrician and asthma specialist who serves on the national board of the American Lung Association, said the AQI is an important measurement but does not necessarily represent every risk a person may face near an industrial fire.

“It does talk about the air quality, and I think it should be taken seriously and it should be monitored,” El-Hasan said. “I don’t think that it is the only monitor.”

Residents should also pay attention to what they smell and how they feel, he said.

“If they can smell something in the air from the fire or in that area, even if the AQI says that everything’s OK, they should still take precautions,” El-Hasan said.

The AQI reports overall air quality using measurements of common pollutants, but an industrial fire can produce a complicated mixture of particles, gases and chemical compounds depending on what materials are burning.

Authorities were still assessing the contents of the Medline warehouse and what substances may have been released. Holt said officials knew that plastics and large quantities of packing materials had burned.

“Among the chemicals that were consumed in the Medline fire were an assortment of respiratory irritants, toxic gases, carcinogens, and at least one neurotoxin,” said Dr. Maggie Park, Public Health Officer, San Joaquin County Public Health Services.

El-Hasan said smoke from burning plastics and medical supplies may create risks beyond the particles typically associated with smoke.

“It’s not just about particles in the air, although that’s very important,” he said. “It’s about the type of chemicals that come from this type of fire that people can be affected by.”

A 2024 review of industrial fires published in the journal Environments found that the pollutants released during a fire depend on the materials involved, the conditions in which they burn and the weather carrying the plume.

The researchers also found that the substances detected can depend on the monitoring equipment used and where monitors are placed. Portable monitors provide officials with rapid information about conditions near affected communities, but they may not identify every individual compound found in a complex smoke plume.

Holt said the district’s monitors measure a range of pollutants, including particulate matter, carbon monoxide, nitrogen oxides and sulfur oxides. The lack of elevated readings suggested that significant concentrations from the plume were not reaching the ground near the monitors.

“Even if there is stuff that burned in that building that we’re not exactly sure of right now, that is not making it down to ground level,” Holt said.

Still, Holt advised residents to stay away from visible or detectable smoke.

“Smoke in and of itself is not good for your health,” he said. “By staying away from the smoke, you’re going to be staying away from any other compounds that might be in that smoke that we haven’t fully assessed yet.”

El-Hasan echoed those sentiments.

Everyone exposed to smoke could be affected, but people with lung or heart disease, young children and older adults face the greatest risk, El-Hasan warned.

“Everyone who is exposed to that smoke is vulnerable in some way,” he said.

Residents should watch for shortness of breath, chest pain, extreme weakness, irritation or burning in the throat or nose, burning eyes and excessive tearing, El-Hasan said.

If you have shortness of breath, he says “you should seek medical care.”

People should also take nausea or other feelings of illness seriously even if public air-quality readings remain within the good or moderate ranges.

“It is always possible that the AQI may not always reflect the level of risk that a person may have when they’re exposed to the air near these fires,” El-Hasan said. “We should trust our senses, our nose. We should trust if we smell something, if we feel ill because of the air in that area.”

Despite the smoke plume being visible from surrounding areas, such as Stockton, the Valley Air District had not detected pollution from the fire affecting the city or other northern portions of San Joaquin County as of Friday.

Forecasting conducted by the district in coordination with the National Weather Service indicated the plume would continue to remain elevated and gradually travel south while spreading out and becoming less concentrated, Holt said.

“The smoke will continue to be in the air,” Holt said. “It will be slowly filtered over the next day or so and spread out, and the concentrations will become much, much less.”

The Valley Air District is working as part of the incident-response team led by the San Joaquin County Office of Emergency Services. It has also coordinated with the California Air Resources Board and sent employees to the fire site Thursday and Friday.

Residents can monitor conditions through the U.S. Environmental Protection Agency’s Fire and Smoke Map, which displays data from government stations and some lower-cost community sensors.

El-Hasan advised people near the fire to keep their windows closed and run their air conditioners when possible. Air cleaners can also reduce exposure, particularly units that include activated carbon filters designed to remove some gases and odors.

Recognizing the cost of cooling and filtering an entire home, El-Hasan said residents could establish at least one room where windows remain closed and the air is cooled and filtered.

People should also minimize outdoor exercise, walking and other strenuous activity when smoke is present, he said. Children should play indoors whenever possible, and residents should check on older relatives and neighbors.

“If they can at least have one room with air conditioning, that would be great,” El-Hasan said. “But they have to close the windows and not let that air in.”

The warehouse blaze is contained for now at the sprawling 1-million-square-foot warehouse, where 35 firefighters are on scene, officials have said. The cause of the fire is still under investigation.

Fire suppression efforts will continue throughout the day Friday and likely extend into the weekend.