The solution in an e-cigarette cartridge, also known as "e-liquid," contains many different, dangerous chemicals such as nicotine, propylene glycol, formaldehyde, and toluene. Of these, the most well known is nicotine, which is also found in tobacco products. Nicotine is highly addictive, and nicotine dependence involves physical and psychological dependence. On its own, its side effects are much more minor compared to those of tobacco smoke, but it is not without adverse effects. Propylene glycol is perhaps the most common ingredient found in e-cigs. The FDA considers this substance as safe to consume in food, but when inhaled, it poses possible breathing problems. This product is an irritant to the eyes and lungs and may cause worsen asthma. Formaldehyde is commonly used to preserve corpses. Exposure to formaldehyde increases the risk of cancer. Often, the amount of this chemical in e-cigarettes is at dangerous levels. Toluene is a volatile organic compound commonly used in industry. Short term exposure can cause nausea, headaches, and fatigue. Long term exposure can cause developmental defects, central nervous system dysfunction, and respiratory tract irritation. Many of the flavorings used in e-cigarettes are recognized as food-grade and safe for consumption, but the effects of inhalation are unknown. It is very possible that these can pose a danger for e-cigarette users.