Plane passengers are used to paying for check-in baggage or priority boarding, but one airport in Venezuela is now charging for the ultimate hidden extra -- air.

Anyone departing from the Simon Bolivar International Airport of Maiquetia in Caracas now faces a levy of 127 bolivars ($18) to pay for a new air conditioning unit installed earlier this year, according to a statement on the airport's website.

The airport says its air conditioning system "eliminates contaminants" and injects ozone into the atmosphere to improve the environment and protect the health of passengers.

It boasts that the system is the first of its kind in an airport anywhere in South America and the Caribbean.

The "breathing tax" which came into force on July 1 and must be paid by all domestic and international passengers to airlines at check in, has generated bemusement in Venezuela, with many taking to Twitter to criticize the measure.

"While the stench of the toilets asphyxiates me ... they have started to charge 127 bolivars for breathing the ozone," 

(CNN)