Airports in Paris, Frankfurt, and Amsterdam are putting increased competitive pressure on London's Heathrow as an international European hub. Although Heathrow is the busiest international airport in the world and the best candidate for expansion by the U.K. government, environmental constraints and community opposition are making it difficult for the U.K.'s Department of Transport to decide where to build a new runway to alleviate congestion. One of the biggest obstacles to building a third runway at Heathrow are the EU's strict rules on air pollution that must be met by 2010. In the meantime, British Airways and Virgin Atlantic argue that if expansion at Heathrow doesn't materialize it will undermine not just their own positions but the national economy as well.