Electronic cigarettes may be more dangerous than previously thought. E-cigarettes contained 1 million times more of a cancer-causing substance than heavily polluted Hong Kong air, a new study from Hong Kong Baptist University found.

Researchers also discovered that the electronic devices contain a type of flame retardant that can affect the reproductive system and could lead to cancer, the South China Morning Post reported Monday. This was the first time that chemical had been found in e-cigarettes.

After these findings were announced, the Hong Kong Council on Smoking and Health, which commissioned the study, called for a ban on e-cigarettes as soon as possible. While these new findings mark the latest point of concern regarding electronic cigarettes, another recent study found that vapor from the devices can contain 15 times more cancer-causing formaldehyde than regular cigarettes, CBS News reported.

The researchers in Hong Kong analyzed 13 types of e-cigarettes and found the level of polycyclic aromatic hydrocarbons (PAHs) — a byproduct of burning petroleum — was higher than expected. The substance contains carcinogenic chemicals and can promote the growth of cancer cells, the South China Morning Post reported.

“[The level of PAHs] in e-cigarettes is at least 1 million times more than roadside air in Hong Kong,” Chung Shan-shan, an assistant professor of biology at Hong Kong Baptist University told South China Morning Post.

The amount of flame retardant — polybrominated diphenyl ethers (PBDEs) — in the e-cigarettes was also much higher than in two different samples of regular cigarettes in the study. The level of PBDEs found could disrupt thyroid hormones and harm the reproductive system. E-cigarettes contain the chemical because it reduces the risk of fire in the devices’ plastic combustibles.

“Even though we don’t know the exact number of e-cigarettes one should take, not to mention that many of the carcinogenic effects are cumulative, I don’t think there is a safe margin,” Chung said.

The e-cigarettes also contain nicotine despite packaging saying they do not contain nicotine or packaging that does not mention the addictive chemical at all. The study was conducted between April and October last year and interviewed 5,252 people.

If Hong Kong were to outlaw e-cigarettes, it would not be the first place to do so. The selling, advertising, import, distribution and manufacture of e-cigarettes have been banned in 16 countries, according to the South China Morning Post. As of Monday, the Hong Kong Food and Health Bureau was discussing a ban and said it hoped to submit a proposal to the legislature soon.