A student holds his CT scan results from the hospital on May 10, 2016. [Photo/IC]

Allegations made by students in Northeast China's Jilin province, who said they were poisoned after being exposed to toxic substances at their school, were called into question on Tuesday when the local government released what it said was proof the school was free of toxins.

The municipal government in Da'an city released a statement on Monday saying an indoor environmental test conducted by a professional company on Sunday proved the student dormitory at Xin'anli Township Junior High School was safe. The move followed a claim by 15 students that they had been poisoned after living and studying in the facilities since last October.

The claims went viral online on Monday after the students collectively posted on Weibo, China's major microblogging platform, that they had been diagnosed with illnesses related to formaldehyde poisoning. They cited symptoms including nausea, coughing and eye infections that they said were diagnosed at the First Hospital of Jilin University.

Students also uploaded photos showing CT scan results from the hospital, and a close-up of a female student's bloodied eye.

They claimed they had been exposed to formaldehyde, a colorless and highly toxic gas that can cause multiple health problems, and even cancer if exposure is long-term.

However, the Da'an government said the school dorm was also tested and found to be toxic free in October 2014, one year after its construction and before the students were moved in.

And Bao Wanguo, director of the infectious disease department at the hospital, told China National Radio on Tuesday that the hospital had never diagnosed the students as suffering from formaldehyde poisoning.

"The students' symptoms, including lung nodules, diarrhea and conjunctiva hemorrhaging, didn't fit in with those related to acute formaldehyde poisoning, but seem more like colds caused by a virus infection," Bao was quoted by CNR as saying on Tuesday.

All 15 students were sent to the hospital by the school for comprehensive medical checks and nine later left with their parents after their symptoms eased, according to the Da'an publicity department.

None of the students or parents responded to interview requests on Tuesday afternoon.

Guo Xiheng, a pulmonary physician from Beijing Chaoyang Hospital, echoed the toxic-free diagnosis.

"Normally, the effect of formaldehyde in construction and decoration materials won't last long enough to be as toxic as claimed in the original post after volatizing for a year before the students moved in," he was quoted as saying by Beijing News on Tuesday.