The growth of social media and waning in-person contact has led Former Surgeon General Vivek Murthy to label loneliness a worldwide epidemic. And it could be lethal.
Julianne Holt-Lunstad, a professor of psychology at Brigham Young University, has found in her research that loneliness reduces people's life spans by the equivalent of smoking 15 cigarettes a day.
Sitting all day increases risk for a raft of different cancers, a 2014 study found.
Researchers included in their meta-analyses data from four million people involving how often they sat to watch TV, do work, and commute.
Each two-hour increase in sitting time upped people's risks for colon, endometrial, and lung cancers, regardless of whether they still exercised during the day.
The Centers for Disease Control and Prevention have called sleep deprivation a public health problem, as some 50 to 70 million people in the US have sleep or wakefulness disorder.
Professor Valery Gafarov, of the World Health Organisation, noted in 2015 that insufficient sleep raises the risk of stroke and heart attack to similar degrees as regular cigarette use.
"Poor sleep should be considered a modifiable risk factor for cardiovascular disease along with smoking, lack of exercise, and poor diet," he said.
Indoor tanning may seem like a more controlled version of regular sunbathing, but both are potentially more dangerous than smoking.
In 2014, researchers published a study in JAMA that found indoor tanning alone led to more cases of skin cancer than smoking did with lung cancer.
"Given the large number of skin cancer cases attributable to indoor tanning, these findings highlight a major public health issue," the investigators wrote.
A wealth of evidence has found that sugary, processed foods high in saturated fats can expose people to potentially fatal diseases at similar, if not greater, rates than smoking.
In 2016, researchers studying the mortality risks from poor diet concluded the death rates exceeded those of alcohol, drugs, unprotected sex, and tobacco combined.