It's an earth-shattering roar that usually has someone within an earshot thinking "do you really have to do that?"

They say they can't help it — it's just how they sneeze.

So how is it that some people can let out a dainty outburst as quiet as a mouse while others sound like they're ripping a hole in the fabric of the universe?

If you're a loud sneezer, you might find the answer a bit on the nose.

What is a sneeze?

A sneeze is believed to be a defence mechanism to protect your airways from noxious fumes, particles and other harmful environmental factors.

Professor Richard Harvey, St Vincents and Macquarie University Hospitals, says it is triggered when the nerves in your nose detect a change in temperature or pain.

That's why things like air conditioning, smoke or the even the sun can make you sneeze.

Professor Harvey says the irritation to your nose makes it watery and itchy, while you also feel the need to hold your breath.

This leads to "an explosive release of air" — the sneeze.

Why do we make noise when we sneeze?

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The sound of a sneeze comes from the air escaping from your mouth or nose.

Professor Harvey says the loudness of a person's sneeze depends on their lung capacity, size and how long they hold their breath for.

"The longer you hold your breath, the more dramatic you make it," he says.

According to Brisbane-based company Noise Measurement Services, an "average" man's sneeze, when recorded from a distance of 60 centimetres, peaks about 90 decibels (dB).

That's a similar level of sound recorded from a lawnmower — a normal conversation is about 60dB.

When the mouth is covered, the sneeze drops to about 80dB.

So can you control how loud you are?

While you can't stop your nose from getting itchy and watery, you can control how loud you sneeze with "higher functions", Professor Harvey says.

He says you can quieten your sneeze by pinching and rubbing the nose or by sneezing through your nose, but that it is a "double-edged sword".

"If you sneeze through your mouth it will be louder, but if you sneeze through your nose it will be wetter, messier," he says.

But how loud you sneeze isn't just an issue of your body and how it works.

Sneezing is messy business. ( CDC: James Gathany )

Dr Barbara Evers, a senior lecturer of sociology at Murdoch University in Western Australia, says the sound of your sneeze also comes down to class, gender and culture.

She says guides on how to behave — known as manners books — have frowned upon poor nasal etiquette since the 15th century.

Dr Evers says over time the advice has changed, meaning the "appropriate" behaviour has become part of normal life and no longer needs to be pointed out — like blowing your nose "on your hat or clothing".

Today, there are still implicit rules around how women can and can't behave, Dr Evers says.

"If you are 'a lady', you do not sneeze very loudly whereas if you are a man you can."

However, she says that is not the case for men in certain cultures like Japan, where loud sneezes are considered rude.

Even in English-speaking countries, saying "ah-choo" is the polite version of a sneeze rather than letting it rip, Dr Evers says.

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What happens if you hold in a sneeze?

Professor Harvey says it's never advised to hold in a sneeze as it is "like a mini explosion in your head and can force air into places it shouldn't go".

"It's quite a forceful respiratory response," he says.

An X-ray shows the hole in the man's throat caused by closing his mouth and nose when he sneezed. ( BMJ: Wanding Yang )

The British Medical Journal last year published the case of a 34-year-old man who ripped a hole in his throat trying to stifle a powerful sneeze.

The British man had attempted to supress a sneeze by clamping his mouth shut and blocking both nostrils.

The resulting force perforated his pharynx, the part of the throat just above the larynx and oesophagus.

He soon found it impossible to swallow without extreme pain and all but lost his voice.

After a week in hospital, during which time he was put on a feeding tube and given intravenous antibiotics to protect against infection, the man was sent home from hospital with the advice not to block his nose when sneezing in the future.

In 2011 it was reported that a 38-year-old man in Boston fractured his larynx when he closed his mouth and pinched his nose during a "vigorous" sneeze.