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The primary base ingredients of the liquid solution is propylene glycol and glycerin. About 20% to 27% of propylene glycol and glycerin-based liquid particles are inhaled. A 2016 study found that 6% of nicotine, 8% of propylene glycol, and 16% of glycerin was breathed out by e-cigarette users. The long-term effects of inhaled propylene glycol has not been studied, and is unknown. The effects of inhaled glycerin are unknown. Being exposed to propylene glycol may cause irritation to the eyes and respiratory tract. When propylene glycol is heated and aerosolized, it could turn into propylene oxide, which the International Agency for Research on Cancer (IARC) states is possibly carcinogenic to humans. The risk from the inhalation of propylene glycol and glycerin is probably low. Propylene glycol and glycerin have not been shown to be safe. Some research states that propylene glycol emissions may cause respiratory irritation and raise the likelihood to develop asthma. Long-term inhalation of propylene glycol indoors could increase risk to children to develop asthma. To lessen the risks, most e-cigarettes companies began to use water and glycerin as replacement for propylene glycol. The inhaled glycerin could cause lipoid pneumonia. Propylene glycol and glycerin had increased the amount of hydrogen peroxide.