Significance

• Someone exposed to secondhand smoke breathes in tobacco's toxic chemicals. Secondary smoke contains 4,000 chemicals, 69 of which can cause cancer, the British National Health Services (NHS) website notes.
  
  

Risks

• Secondhand smoke is more toxic than smoke inhaled directly because it comes from the lit ends of cigarettes and cigars and is unfiltered. Children are particularly at risk from passive smoking, as their bodies are still developing. They are more likely to develop breathing problems and inner ear infections.
  
  

Effects

• The Green Facts website says that studies have shown a link between passive smoking and the risk of developing lung cancer. The risk rises with increasing exposure. "This evidence is sufficient to conclude that involuntary smoking is a cause of lung cancer in never-smokers," the website says.