While several buildings had been evacuated in the wake of the blast, de Blasio warned those still in the area to limit exposure by "staying indoors and if possible keeping their windows closed."
"If you’re right up on the site or a block away in any direction of the air quality you should avoid it as you can," de Blasio said. "If you’re right in the immediate surrounding blocks keep your windows shut."
The mayor, joined by officials at City Hall, said the air could be a problem for those with asthma or other respiratory illnesses, but added that the general population would not be at risk.
"The conditions we see out there, as of this moment, are not such as to cause significant health problems for healthy people, but they can have an undue effect on vulnerable populations like seniors and children," de Blasio said, adding that anyone who experiences shortness of breath or a worsening medical condition should seek help immediately.
De Blasio also noted that the Department of Environmental Protection has not found indications of asbestos in the air.
The city referred New Yorkers looking for more information to the Department of Health's website on the effect of fire and smoke on air quality.
Air quality has been a tremendous concern for East Harlem residents following the blast that leveled two Park Avenue apartment buildings Wednesday morning, killing seven people and injuring more than 60. The New York Post reported that many residents in the area bought face masks out of fear of breathing in the smoke.
Environmental advocates could not speculate as to the makeup of the smoke.
"Unfortunately without doing some monitoring it would be impossible to say," said Peggy Shepherd, executive director of the Harlem environmental watchdog WE ACT for Environmental Justice. "We’d also have to know what buildings materials were burned."
"It really does depend on what burned," added Dr. Rachel Miller, professor of medicine in pediatrics and environmental health sciences at Columbia University Medical Center. "Some things burn dirtier than others."
"I think the most sound thing to do is to contact your doctor if you're feeling sick," Miller said.
Glenn Corbett, a professor at John Jay College and a member of the Fire Code Advisory Council for New Jersey, also backed the city's advice, saying that although he did not know the specifics of the buildings, the smoke was most likely not a huge public threat.
"I think this fire is in all likelihood ordinary combustibles," Corbett said. "You had two entire buildings on fire even though they were collapsed."
Schools in the area opened normally Thursday, and Department of Education officials denied reports of early dismissals due to poor air quality.
"No schools are closed. We have taken air samples and readings are well below guidelines," a DOE spokeswoman said.