When you smoke a cigarette (or roll-up, pipe or cigar), most of the smoke doesn't go into your lungs, it goes into the air around you where anyone nearby can breathe it in.
Secondhand smoke is the smoke that you exhale plus the "sidestream" smoke created by the lit end of your cigarette.
When friends and family breathe in your secondhand smoke – what we call passive smoking – it isn't just unpleasant for them, it can damage their health too.
People who breathe in secondhand smoke regularly are more likely to get the same diseases as smokers, including lung cancer and heart disease.
Pregnant women exposed to passive smoke are more prone to premature birth and their baby is more at risk of low birthweight and cot death.
And children who live in a smoky house are at higher risk of breathing problems, asthma, and allergies.
How to protect against secondhand smoke
The only way to protect your friends and family from secondhand smoke is to keep the environment around them smoke free.
The best way to do that is to quit smoking completely. If you're not ready to quit, make every effort to keep your cigarette smoke away from other people and never smoke indoors or in the car.
- always smoke outside
- ask your visitors to smoke outside
- don't smoke in the car or allow anyone else to
Take steps NOW to stop smoking.
The risks of passive smoking
Secondhand smoke is a lethal cocktail of more than 4,000 irritants, toxins and cancer-causing substances.
Most secondhand smoke is invisible and odourless, so no matter how careful you think you're being, people around you still breathe in the harmful poisons.
Opening windows and doors or smoking in another room in the house doesn't protect people. Smoke can linger in the air for two to three hours after you've finished a cigarette, even with a window open. And even if you limit smoking to one room, the smoke will spread to the rest of the house where people can inhale it.
Is passive smoking harmful?
Read about stop smoking treatments.
Children and passive smoking
Passive smoking is especially harmful for children as they have less well-developed airways, lungs and immune systems.
It's estimated that more than one in five children in the UK live in a household where at least one person smokes and, as a result, they're more likely to develop:
Children are particularly vulnerable in the family car where secondhand smoke can reach hazardous levels even with the windows open.
It's estimated that smoking in cars produces concentrations of toxins up to 11 times higher than you used to find in the average smoky pub.
To protect children, there is a new ban on smoking in cars and other vehicles carrying children. From October 1 2015 it is against the law to smoke in a private vehicle if there’s a young person under-18 present.
Read about the new law on smoking in private vehicles.
How safe is e-cig vapour?
E-cigarettes don't produce tobacco smoke so the risks of passive smoking with conventional cigarettes don't apply to e-cigs.
Research into this area is ongoing, but it seems that e-cigs release negligible amounts of nicotine into the atmosphere and the limited evidence available suggests that any risk from passive vaping to bystanders is small relative to tobacco cigarettes.
In England, the Government has no plans to ban vaping indoors (although some employers have banned them in the workplace) but some health professionals recommend avoiding using them around pregnant women, babies and children.
Read about the safety of e-cigarettes.
Your GP can give you advice about quitting smoking.