'What if I just started shouting?' - when to worry about intrusive thoughts

Have you ever sat in a boring meeting and wondered: "What if I just started shouting?" Or you're driving along and you think: "What if I crash?"

These unsettling scenarios are known as "intrusive thoughts" which the majority of us will experience from time to time and feel able to shrug off.

But for some they can become overwhelming obsessions which lead to compulsive behaviours.

When Dr Nina Higson-Sweeney was a child, she was convinced that unless she only had "good" thoughts the whole way home from school, her family would be harmed.

"If I had an intrusive thought, I'd restart the walk from the bus stop," she says. "I was genuinely terrified that if I didn't redo it and something happened, it would be my fault".

Nina was diagnosed with obsessive compulsive disorder (OCD) aged 10 and now works as a psychology researcher at the University of Oxford where she specialises in child and adolescent mental health.

"Obsessions are intrusive and unwanted thoughts, feelings and sensations while compulsions are repeated, ritualised acts that are done to neutralise or alleviate the anxiety caused by the obsessions," Nina tells the BBC's Complex podcast.

Around 1- 4% of the population is thought to live with OCD but the number of 16-24 year olds in England reporting symptoms of it has more than tripled in a decade.

The condition is now the second-most widespread mental health disorder for young adults, a major NHS England survey suggests.

Intrusive thoughts can be extremely distressing and often focus on topics that feel completely at odds with a person's values or identity.

"You might have thoughts about harm coming to loved ones," says Nina.

"It could be questioning one's sexual attraction, thinking are you homosexual? Are you heterosexual? It can even be as extreme as worrying are you a paedophile?

"A really common one is intrusive thoughts about contamination and worries about getting ill or spreading sickness," she says.

OCD most commonly begins in puberty or early adolescence, but some people are diagnosed later in life as they can "go years masking or covering up the distress," she says.

Research suggests there may be a genetic component to developing OCD as well as links to early life stress such as bullying, bereavement or family breakdown, she says.

Chartered psychologist Kimberley Wilson says almost everyone experiences intrusive thoughts at some point.

"Research suggests around 80% of us have these thoughts," she explains.

For most people, those thoughts pass quickly.

"We can look at them, think they're weird, and put them aside," she says.

If you're unable to dismiss the thoughts, that's when you may need to seek help, she suggests.

"OCD-related thoughts don't pass on, they take up residence and they're never positive thoughts - they're aggressive, hostile and not easy to deal with. That's when it becomes all-consuming and leads to compulsions."

Signs of compulsions can be mental, like counting to a certain number, or visible, such as repeatedly checking car tyres even when you know they're fine.

In these cases "a professional or specialist can work out what's best for you," says Nina.

Alongside professional help, Nina says there are techniques people can use day-to-day to reduce distress.

One is learning to label thoughts. "Recognising 'I'm having an intrusive thought' creates distance and remind me that it's not me," she explains.

Some people also find it helpful to visualise OCD as something separate.

"Drawing what OCD looks like can help - there's me and there's OCD, and those are two different things."

Self-care also matters. "Eating well, resting and physical activity can help as my OCD is always worse when I'm stressed and not looking after myself," she says.

Today, Nina still lives with OCD but has learned how to manage it.

"I've never got over OCD, but I can function with it. I now have mild intrusive thoughts and a lot of insight into how I manage it. When I'm stressed though they're harder to dismiss and can still lead to compulsions."

• If you've been affected by issues in this story, details of organisations offering advice and support for people in the UK are available from BBC Action Line