The right location has always been a constant criterion in the determination of successful airport hubs. But politics and technology can change those geographic criteria over time. A gateway airport is one that serves primarily as a departure point for international flights. While the gateway airport can resemble a hub in that it functions to connect passengers, the focus will be to funnel those passengers to long-distance international flights. Commonly, there will be more domestic flights using smaller airplanes that will transfer those passengers onto fewer airplanes of larger size, often wide bodies, capable of operating long-distance operations. London Heathrow and Tokyo Narita airports are good examples of major gateways. While these airports remain major intercontinental gateways, competing airports and airlines have more options as long-range aircraft have enabled airlines to bypass the hubs and fly directly to interior destinations. The presence of smoke and dust in an airport environment can be the result of man-made or natural phenomena.