have a wide range of budgets and tastes, and a wide variety of resorts and hotels have developed to cater for them. For example, some people prefer simple beach vacations, while others want more specialized holidays, quieter resorts, family-oriented holidays, or niche market-targeted destination hotels. The developments in air transport infrastructure, such as jumbo jets, low-cost airlines, and more accessible airports have made many types of tourism more affordable. A major factor in the relatively low cost of air travel is the tax exemption for aviation fuels. The WHO estimated in 2009 that there are around half a million people on board aircraft at any given time. There have also been changes in lifestyle, for example, some retirement-age people sustain year-round tourism. This is facilitated by internet sales of tourist services. Some sites have now started to offer dynamic packaging, in which an inclusive price is quoted for a tailor-made package requested by the customer upon impulse. There have been a few setbacks in tourism, such as the September 11 attacks and terrorist threats to tourist destinations, such as in Bali and several European cities. Also, on 26 December 2004, a tsunami, caused by the 2004 Indian Ocean earthquake, hit the Asian countries on the Indian Ocean, including the Maldives. Thousands of people died including many tourists. This, together with the vast clean-up operations, stopped or severely hampered tourism in the area for a time. Individual low-price or even zero-price overnight stays have become more popular in the 2000s, especially with a strong growth in the hostel market and services like CouchSurfing and airbnb being established. There has also been examples of jurisdictions wherein a significant portion of GDP is being spent on altering the primary sources of revenue towards tourism, as has occurred for instance in Dubai. Sustainable tourism Ecotourism Ecotourism, also known as ecological tourism, is responsible travel to fragile, pristine, and usually protected areas that strives to be low-impact and (often) small-scale. It helps educate the traveller; provides funds for conservation; directly benefits the economic development and political empowerment of local communities, and fosters respect for different cultures and for human rights. Take only memories and leave only footprints is a very common slogan in protected areas. Tourist destinations are shifting to low carbon emissions following the trend of visitors more focused in being environmentally responsible adopting a sustainable behavior. Volunteer tourism Volunteer tourism (or voluntourism) is