Relax: the aliens aren't using poisonous space rocks to soften us up in advance of an invasion, after all.

Last week, you may recall, a meteorite fell in Peru. Locals who were first on the scene reported feeling ill immediately after contact with the space rock, with symptoms including headaches and nausea.

According to Luisa Macedo, a researcher for Peru's Mining, Metallurgy, and Geology Institute (INGEMMET), the cause of the illness was arsenic poisoning.

Speaking to National Geographic, Macedo explains that numerous arsenic deposits exist in the soils of Peru. The most likely explanation is that these deposits have contaminated the local water supply, which was vaporised when the meteorite landed.

Astronomer José Ishitsuka notes: "If the meteorite arrives incandescent and at a high temperature because of friction in the atmosphere, hitting water can create a column of steam."

The early arrivals on the scene would then inhale this water vapour, and with it small amounts of arsenic. Although arsenic can be deadly in large doses, small doses are easily survivable, provided they are not repeated. Prolonged exposure can lead to digestive problems, cancer, diabetes, liver disease and thickened skin.

Fortunately, all those who had complained of feeling sick were on the mend by last Wednesday, Macedo said. ®

Related stories

• German lad hit by 30,000 mph meteorite (12 June 2009)
• Astronauts suffer 'exploding' space headache (2 June 2009)
• UK's biggest meteorite impact rocked Scotland (26 March 2008)
• Toxic chemicals suit dumped on IBM (4 January 2008)
• Peruvian 'meteorite' strike provokes noxious gas attack (18 September 2007)
• Bubbly meteor hints at seeds of life (4 December 2006)
• Lack of proper kip killed dinosaurs (22 March 2006)
• Frozen Mars bars possibility of life (22 July 2005)
• Bruce Willis still on Armageddon alert (19 April 2005)
• Opportunity sniffs out meteorite on Mars (19 January 2005)
• Mars rock found in Antarctica (21 July 2004)