British teenagers are exposed to a high level of images of alcohol and tobacco in YouTube music videos, research suggests.
Those aged 13 to 15 - and girls - suffer the most exposure to such images, which have been shown to increase the chance youngsters will start smoking or drinking, experts argue.
Writing in the Journal of Epidemiology and Community Health, they said relatively little attention had been paid to YouTube content, which was hugely popular.
There were also few controls on digital music videos, according to the team, which included experts from the UK Centre for Tobacco and Alcohol Studies.
Researchers analysed the 32 most popular music videos of top 40 chart songs in the UK during a 12-week period between November 2013 and January 2014.
They worked out the total number of images, depictions and lyrics (impressions) of alcohol and tobacco content.
Surveys of more than 2,000 adults and 2,000 youngsters aged 11 to 18 were carried out in March 2014 to see how many had ever seen each of the 32 videos.
Overall, the videos produced 1,006 million impressions of alcohol and 203 million of tobacco.
Most of this content was delivered to 25 to 34-year-olds, but levels of individual exposure were almost four times higher among teens, the data showed.
Teens aged 13 to 15 received an average of 11.48 tobacco impressions, while those aged 16 to 18 received an average of 10.5. The figure was 2.85 for adults.
Exposure was around 65% higher among girls.
The highest numbers of tobacco impressions were to 13 to 15-year-olds.
An estimated 52.11 alcohol impressions were delivered to each teenager, compared with 14.13 to each adult, the study found.
Individual exposure levels increased to 70.68 among 13 to 15-year-old girls.
The experts said: "If these levels of exposure were typical, then in one year, music videos would be expected to deliver over four billion impressions of alcohol, and nearly one billion of tobacco in Britain alone.
"Further, the number of impressions has been calculated on the basis of one viewing only; however, many of the videos had been watched multiple times, so this number is likely to be much bigger."
The researchers added: "In the UK, paid-for placement of tobacco products in music videos is prohibited by the Tobacco Advertising and Promotion Act 2002, but this law does not apply to videos produced outside the UK.
"Alcohol promotion is much less tightly controlled, being subject only to a code of conduct issued by the independent Advertising Standards Authority (ASA)."
A spokeswoman for the ASA said: "No evidence has been provided to us to suggest that portrayals of alcohol and tobacco in music videos are paid for by advertisers.
"Purely editorial content of programmes or music videos is not classed as advertising under the UK Advertising Code and therefore falls outside the remit of the ASA.
"The UK has some of the strongest alcohol advertising rules in the world and, when ads do fall within our remit, the ASA will not hesitate to ban any that break the rules.
"The strict alcohol advertising rules are mandatory and place a particular emphasis on protecting young people from harm."