(CNN) - To better understand the vaping epidemic in high schools across the United States, researchers have turned to the ground.
Researchers in Northern California have "systematically scanned the student parking lots and exterior school perimeter areas" to collect e-cigarette, tobacco and cannabis waste on the ground, according to a note published on Thursday in the US Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report.
There were differences in what was found on the ground at schools with a predominantly middle- and upper-income student population compared with schools with a lower-income student population.
"E-cigarette waste and combustible tobacco product waste contaminate the Bay Area high schools studied and confirm use of these products by high school students," wrote researchers Jeremiah Mock, of the University of California, San Francisco, and Yogi Hendlin, of Erasmus University in the Netherlands, who co-authored the note.
"Cannabis product waste represents an emerging issue," they wrote.
The note detailed how contamination from e-cigarette product waste, tobacco product waste and cannabis product waste was studied at 12 public high schools in Alameda, Contra Costa, Marin and San Francisco counties between July 2018 and April 2019.
Across those schools, there were a total of 18,831 students enrolled, and overall 893 waste items were collected, among which 19% came from e-cigarette products. Nearly all of those products were Juul or Juul-compatible pods and pod caps, according to the note.
The note described that almost all Juul or Juul-compatible pods and caps were found at schools with predominantly middle- and upper-income student populations.
Overall, the note indicated that 64% of pod caps were from mint-flavored and other menthol-flavored pods.
The researchers conducted additional scans at one school in an upper-income area three months after e-cigarette maker Juul announced it was stopping retail sales of flavor products, except Cool Mint and Classic Menthol. Those additional scans yielded 172 mint, 20 mango and four fruit pod caps, as well as three yellow caps, which could have been banana or mango flavors, according to the note.
At four high schools with predominantly lower-income students, eight e-cigarette product waste items and 71 little cigar or cigarillo plastic wrappers and mouthpieces were collected, among which 94% were flavored, according to the note.
"The large proportions of flavored products identified in this study are consistent with findings from other studies showing high prevalence rates of flavored e-cigarette and combustible tobacco product use" among youth, Mock and Hendlin wrote in the note.
Across all schools, 620 cigarette butts and 14 cannabis product waste items were collected, including vaporizer pens, cartridges and packaging "from high-potency pineapple- and lemon-flavored cannabis oil concentrate vaporizer cartridges," according to the note.
More than 3.6 million middle and high school students in the United States have said that they use e-cigarettes, according to the 2018 National Youth Tobacco Survey. Use of any flavored e-cigarette went up among those young users between 2017 and 2018, rising from 60.9% to 67.8%, and menthol use increased from 42.3% to 51.2%, according to the survey.
Several states have taken action to prohibit the sale of certain flavored e-cigarette products, in an effort to curb teen vaping, and the US Food and Drug Administration is expected to finalize a policy in the coming weeks that would clear the market of flavored e-cigarettes and vaping products, Trump administration officials have said.
The findings in the note are limited to only those high schools in the San Francisco Bay Area, and more research is needed to determine whether similar findings would emerge in other parts of the United States.
"Further research and actions at national, state, and community levels are needed to inform policymaking to reduce youth access to and use of tobacco products, including e-cigarettes, and cannabis products. Youth use of flavored tobacco products, including mint and all other mentholated flavors, is of particular concern," Mock and Hendlin wrote in the report.
"Likewise, measures are needed to eliminate environmental contamination from e-cigarette, combustible tobacco product, and cannabis product waste in and around schools," they wrote. "Schools can engage students in garbology projects to identify existing and new use of these products and to raise awareness about their hazardous health and environmental impacts."
Dr. Edith Bracho-Sanchez, a primary care pediatrician and assistant professor of pediatrics at Columbia University in New York, said that she was not surprised by the amount of waste related to both e-cigarette and traditional tobacco products the researchers found at high schools in the Bay Area.
"We know we have an ongoing vaping epidemic in teens and we know that teens who vape are more likely to use other tobacco products in the future," Bracho-Sanchez said in an email on Thursday.
"I applaud these researchers for quantifying the extent of the issue in this way since self-reporting surveys have limitations and the sleek, small nature of some of these devices also means they often go unperceived by both teachers and parents," she said.
She added that she also was not surprised that high schools in lower-income neighborhoods had greater amounts of waste from traditional combustion cigarette products -- "not only are they more affordable, we know that kids in these neighborhoods are specifically targeted by advertisers," she said.
The report comes at a time when the United States continues to face an outbreak of lung injuries associated with e-cigarette use or vaping. The specific chemical exposure causing these lung injuries remains unknown, according to the CDC.
As of Tuesday, there were 1,299 lung injury cases associated with e-cigarette products in 49 states, the District of Columbia and the US Virgin Islands, according to the CDC. Alaska is the only state without a reported case. About 80% of patients are under 35 years old, and they range in age from 13 to 75 years. States have reported 27 vaping-related deaths. The CDC, US Food and Drug Administration, state and local health departments and other clinical and public health partners are investigating the multistate outbreak.