The electronic cigarette was invented in China in 2004 by pharmacist Hon Lik. It consists of a battery, cartridge containing liquid (often with nicotine), and atomizer. When used, it produces an aerosol that some studies have found contains toxic chemicals and particles, though at lower levels than cigarette smoke. Nicotine in e-cigarettes is highly addictive and can be toxic at low levels, especially for youth. Major tobacco companies have entered the e-cigarette market due to declining cigarette sales. While e-cigarettes may contain fewer toxins than cigarettes, they are not harmless.