Watching TV for an average of six hours a day could shorten the viewer's life expectancy by almost five years, indicates research published online in the British Journal of Sports Medicine.
The impact rivals that of other well known behavioural risk factors, such as smoking and lack of exercise, the study suggests.
Sedentary behaviour - as distinct from too little exercise - is associated with a higher risk of death, particularly from heart attack or stroke. Watching TV accounts for a substantial amount of sedentary activity, but its impact on life expectancy has not been assessed, say the authors.
They used previously published data on the relationship between TV viewing time and death from analyses of the Australian Diabetes, Obesity and Lifestyle Study (AusDiab), as well as Australian national population and mortality figures for 2008, to construct a lifetime risk framework.
AusDiab is a national survey of a representative sample of the population, starting in 1999-2000, and involving more than 11,000 adults aged 25 or older.
The authors then constructed a risk framework for the Australian population in 2008, based on the answers the survey participants had given, when quizzed about the total amount of time they had spent in the previous week watching TV or videos.
In 2008 the authors estimated that Australian adults aged 25 and older watched 9.8 billion hours of TV, which led them to calculate that every single hour of TV watched after the age of 25 shortened the viewer's life expectancy by just under 22 minutes.
Based on these figures, and expected deaths from all causes, the authors calculated that an individual who spends a lifetime average of six hours a day watching TV can expect to live just under five fewer years than someone who does not watch TV.
These figures compare with the impact of other well known lifestyle factors on the risk of death from cardiovascular disease after the age of 50, including physical activity and obesity.
For example, other research has shown that lifelong smoking is associated with the shortening of life expectancy by more than 4 years after the age of 50, with the average loss of life from one cigarette calculated to be 11 minutes - equivalent to half an hour of TV watching, according to the authors' risk framework.
Their findings "suggest that substantial loss of life may be associated with prolonged TV viewing," say the authors. And they add: "While we used Australian data, the effects in other industrialised and developing countries are likely to be comparable, given the typically large amounts of time spent watching TV and similarities in disease patterns."
They conclude: "If these [figures] are confirmed and shown to reflect a causal association, TV viewing is a public health problem comparable in size to established behavioural risk factors."