As the party conference season gets under way in the UK, research in Emergency Medicine Journal shows that lobbing raw eggs at people as a harmless form of protest or prank can actually result in serious eye injury.

Between November 2004 and December 2005 researchers at one specialist department monitored more than 18,000 patients requiring eye treatment.

Of these, 13 patients had sustained injuries as a result of being assaulted by a raw egg. Twelve of the patients were men, whose average age was 27.

Most of the cases were clustered in October, during the period around Halloween.

In nine people, the left eye was injured. In only one person was eyesight unaffected by the incident.

Three people’s injuries were classified as “minor,” amounting to bruising and bloodshot eyes. The injuries were classed as moderate in a further two cases; the remainder were classified as serious.

The serious injuries included tears and severe bruising to the retina, disruption of the light receptor cells in the eye, and raised internal pressure, which can damage the optic nerve and cause permanent loss of eyesight, and damage to the macula.

Twelve patients recovered, one of whom needed major surgery to do so.

The authors comment that damage to the eye as a result of being hit by a blunt object, referred to as “blunt ocular trauma,” makes up around a third of injuries seen in primary care eye clinics.

“The dimensions of an egg are similar to those of a squash ball, with a considerably greater weight, meaning that eggs as missiles can easily fit into the orbital rim, causing severe blunt injury even when thrown by hand,” write the authors.

The belief that this prank is harmless, is misplaced, they warn.