Without doubt, war correspondents are wise to wear helmets and flak jackets while sending dispatches from shell-pounded streets crackling with gunfire.

But when Sky News reporter Alex Crawford rode into Tripoli this week on a rebel pickup truck, she was more worried about celebratory gunfire – because of the danger posed by bullets landing on her head.

While some early studies conjectured that bullets shot upwards simply vanished into space, the threat from falling ordnance is not to be dismissed.

To explore the finer details of the rule that what goes up must come down, Benjamin Robins reported in 1761 that a largebore bullet fired upwards returned to Earth three quarters of a mile away and half a minute later. More precise studies followed.

In the autumn of 1910, an RL Tippins hauled a Maxim machine gun to the tidal mud of the river Stour in Suffolk and fired two bursts of about 30 bullets straight up into the air.

With stopwatch in hand, he timed the bullets as they fell to Earth, recording flight times of about 55 seconds.

The conclusion from the experiments was that the bullets reached 2,750 metres (9,000ft), taking 19 seconds to rise and almost twice as long – 36 seconds – to fall.

Recalling the experiment, Tippins said: "We had no head cover, but trusted to the wind to carry them away far enough to miss us."

But what do we know today of the risk of injury from falling bullets?

Between 1985 and 1992, a group of physicians at the Martin Luther King/Drew medical centre in Los Angeles studied victims of gunshot wounds and identified 118 thought to have been hit by falling bullets. These included people struck by bullets while going about their everyday business, far away from known gunfire.

Only six of the regular bullets were traced by the police, to shootings that occurred up to a mile away. But some people were hit by high velocity rounds whose sources were never traced.

For those hit by falling bullets, the chance of the wound being fatal was far higher than a typical shooting. The hospital put deaths from regular shootings between 2% and 6%, while for those struck by falling bullets the death rate was close to one third.

The reason was simple – those unlucky enough to be hit by falling shells were more likely to be struck on the head, and while the bullets were travelling slower after being shot into the sky, they were still capable of smashing skulls.

According to the doctors, a spent bullet falls back to Earth with a speed of between 90 and 180 metres per second. A bullet travelling at less than 60 metres per second can cause a fatal skull injury, they added.

The hazard posed by celebratory shots led Los Angeles city council to ban the firing of bullets into the air in 1989. Later, sales of bullets in the runup to New Year's Eve were prohibited.

The issue was picked up in 2003 by the US Centres for Disease Control and Prevention, which raised alarm over the dangers of falling bullets discharged during new year celebrations in Puerto Rico in 2003/4. They logged 19 injuries, including one death, over two days.

The perils of falling bullets are not confined to war zones and inner cities. In 2008, American chef Paul Prudhomme was setting up his cooking tent on a golf range ahead of the PGA tour in New Orleans when he flinched from what he thought was a bee sting.

When he shook his shirt sleeve, a .22 calibre bullet dropped out. According to police, it could have been fired one and a half miles away. Prudhomme continued to cook with a hole in his chef's coat and a cut to his arm.

The danger of falling bullets, from celebrations linked to New Year's Eve, weddings and religious festivals, has prompted governments around the world to launch educational programmes to discourage trigger-happy partygoers.

A TV and radio campaign in Macedonia in 2005 used the slogan: "Bullets are not greeting cards – celebrate without weapons."