When thousands of visitors to London next week line the official royal wedding parade route, they may find their breath taken away by more than just the site of a newly minted princess in a horse-drawn carriage.

On April 21, the U.K.'s capital city exceeded the annual limits set by the European Union for air pollution breaches. Since the start of the year, London has violated the accepted amount of air particle pollution 36 times. This air pollution is caused generally by vehicles, factories and construction.

According to the U.K. Environmental Audit Committee, air pollution from traffic and industry kills as many as 50,000 people in the U.K. a year.

With London's air pollution woes likely to continue into next week, Green Party London Assembly Member and mayoral candidate Jenny Jones urged Mayor Boris Johnson to issue a royal wedding smog alert.

"We have the prospect of hundreds of thousands of visitors flocking to central London, to stand around all day in one of the most beautiful but polluted cities in Europe," she said in her letter. "The mayor should be making people aware of what they are breathing in and the risks to their health. It is important information for people to know if they suffer from a pre-existing respiratory problem. London is currently experiencing an air pollution episode and this is likely to carry through to the royal wedding."

Beyond the embarrassment of a dirty, smog-filled setting in front of an international audience, London also risks a $500 million fine from the EU for its repeated air quality violations.

Visitors to the capital are being urged to sign up for a free air-monitoring service that sends cellphone text updates about pollution levels. A quick glance at the site shows the entire downtown area labeled as "high air pollution everywhere" — a clear sign that this might be one event that's best watched online.

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.

A royal wedding smog alert
Mayor of London pressured to issue air pollution alert as thousands descend on the capital for Prince William and Kate Middleton's nuptials.