2019–2020 vaping lung illness outbreak

An outbreak of e-cigarette, or vaping, product use-associated lung injury (EVALI) started in 2019 among users of illegal, unregulated cannabis vaping products, almost exclusively in the United States. The first cases of this particular outbreak were identified in Illinois and Wisconsin in April 2019; as of 18 February 2020, a total of 2,807 hospitalized cases, including 68 deaths, have been confirmed. According to the U.S. Centers for Disease Control (CDC), "Vitamin E acetate is strongly linked to the EVALI outbreak...Evidence is not sufficient to rule out the contribution of other chemicals of concern, including chemicals in either THC or non-THC products, in some of the reported EVALI cases".

Cases peaked in September 2019, and have slowly declined since. The CDC stopped reporting on EVALI cases in February 2020, due to the decline in cases, but as of December 2020, continues to monitor cases that may arrive in emergency departments. Some states continue to record new EVALI cases. As of January 2022, the state of California has reported at least 40 cases on their update page, that were diagnosed after February 2020. According to an article in the Radiological Society of North America, news published in March 2022, EVALI cases continue to be diagnosed. "EVALI has by no means disappeared," Dr. Kligerman said. "We continue to see numerous cases, even during the pandemic, many of which are initially misdiagnosed as COVID-19." There have been at least 73 cases diagnosed in Utah after the last CDC update of February 2020.

CDC investigators identified direct exposure to chemicals present in illegal cannabis vaping products as a likely culprit for the outbreak, but the CDC did not rule out other chemicals in nicotine vapes as possible causes. Vitamin E acetate is a very strong culprit of concern in the lung illnesses related to THC-based vaping products but according to the CDC, "No specific e-cigarette device or substance has been linked to all cases, and e-cigarettes include a variety of chemical and additives; consumers may not know what these products contain." Though patients have reported using vaping products containing THC, nicotine, or both types, 84% of patients studied by the CDC admitted THC use. The majority of those affected were young adults aged 18–34, which is the group with the greatest prevalence of cannabis use in the U.S.

Nicotine-containing products are regulated in the U.S. by the Food and Drug Administration (FDA) which has not approved any vape product for sale; THC products are illegal under federal law, but allowed and regulated by many states. Prior to the outbreak, vitamin E acetate was used in low concentrations, lower than 20% of the formula in vape cartridges, as a thickening agent. As a result of a limited availability of cannabis in California, as well as high demand, illicit sellers had used about 50% or higher of diluent thickeners in their formulas to bulk up tiny potency vape cartridges to create the illegal and dangerous products, and vitamin E acetate is a common choice because it resembles THC oil. Some jurisdictions took action to restrict the sale of products containing vitamin E acetate and other chemicals in response to the outbreak, but THC products in states where it is illegal are produced unlawfully (and sometimes obtained illegally in jurisdictions where legal, by uninformed consumers).

In 2021, researchers at Johns Hopkins University analyzed the vape aerosols of popular brands such as Juul and Vuse, and found "nearly 2,000 chemicals, the vast majority of which are unidentified." The CDC has not ruled out nicotine vapes as a possible additional cause of some cases of EVALI.