More than two-thirds of around 1.2 million deaths of adolescents due to preventable causes such as road accident injuries and suicide took place in low- and middle-income countries including India in 2015, the WHO has said.
A WHO report said that most of these deaths could be prevented with good healthcare services, education and social support, and recommended introduction of comprehensive sexuality education in schools and setting higher age limits for alcohol consumption.
"More than 3,000 adolescents die every day, totalling 1.2 million deaths a year, from largely preventable causes.
"In 2015, more than two-thirds of these deaths occurred in low- and middle-income countries in Africa and Southeast Asia," the report by the WHO and its partners said.
It said road accident injuries, lower respiratory infections and suicide are the biggest causes of death among adolescents.
"Adolescents have been entirely absent from national health plans for decades.
"Relatively small investments focused on adolescents now will not only result in healthy and empowered adults who thrive and contribute positively to their communities, but it will also result in healthier future generations, yielding enormous returns," said Flavia Bustreo, the WHO Assistant Director-General.
Data in the report -- Global accelerated action for the health of adolescents (AA-HA!): Guidance to support country implementation -- revealed stark differences in causes of death when separating the adolescent group by age (younger adolescents aged 10-14 years and older ones aged 15-19 years) and by sex.
The report also includes the range of interventions -- from laws for wearing seat belts to comprehensive sexuality education -- that countries can take to improve their health and well-being and dramatically cut unnecessary deaths.
Road accident injury was the leading cause of adolescent death among 10-19 year olds in 2015, resulting in approximately 1,15,000 adolescent deaths.
Older adolescent boys aged 15-19 years experienced the greatest burden and most young people killed in road crashes are vulnerable road users such as pedestrians, cyclists and motorcyclists, the report said.
"However, differences between regions are stark. Looking only at low- and middle-income countries in Africa, communicable diseases such as HIV/AIDS, lower respiratory infections, meningitis, and diarrhoeal diseases are bigger causes of death among adolescents than road injuries," it said.
Noting that the picture for girls differs greatly, the report said that the leading cause of death for younger adolescent girls aged 10-14 years are lower respiratory infections such as pneumonia -- often a result of indoor air pollution from cooking with dirty fuels.
Pregnancy complications, such as haemorrhage, sepsis, obstructed labour and complications from unsafe abortions, are the top cause of death among 15-19-year-old girls, the report said.
"Suicide and accidental death from self-harm were the third cause of adolescent mortality in 2015, resulting in an estimated 67,000 deaths.
"Self-harm largely occurs among older adolescents, and globally it is the second leading cause of death for older adolescent girls. It is the leading or second leading cause of adolescent death in Europe and South-East Asia," the report said.
"Improving the way health systems serve adolescents is just one part of improving their health.
"Parents, families, and communities are extremely important, as they have the greatest potential to positively influence adolescent behaviour and health," said Anthony Costello, Director of the WHO's Maternal, Newborn, Child and Adolescent Health.
The 'AA-HA! Guidance' recommends interventions across sectors, including comprehensive sexuality education in schools, higher age limits for alcohol consumption, mandating seat belts and helmets through laws, reducing access to and misuse of firearms, and reducing indoor air pollution through cleaner cooking fuels.