access to air services as an important factor in the decision of where in the UK to locate operations, whilst one in ten companies report that the absence of good air transport links has affected their decision to invest in the UK. The survey has been criticised as suffering from a poor response rate and therefore open to bias, though this issue has been recognised and rationalised by the report's authors. Tourism is an industry where the influence of air transport services is more obvious. In 2005 some 22 million overseas visitors arrived by air, spending around £12 billion (1.1 per cent of GDP) and supporting 170,000 jobs in the tourist industry. In the same year air travel also accounted for 36 million trips abroad by UK tourists, and UK tourists as a whole spend twice as much abroad as overseas visitors spend in the UK. This has led to the assertion that aviation represents a "net negative effect" on the UK tourism industry, and that restraining demand for air travel would encourage more domestic tourism, with the consequent economic benefit of reducing the tourism deficit. Exports and imports by air in 2005 were estimated at £62.7 billion and £59.6 billion respectively, with a significant majority of air freight operations being conducted with countries outside of the EU, and express freight operations transporting 5 per cent by value of all UK exports in 2004. Whilst export/import facilities provide opportunities for international trade and competition, they are not without negative effect, and British horticulture is one example of domestic industry damaged by cheap imports. Forecast economic outlook Attempts to quantify the economic effects of growth in the air transport sector generate results which depend on assumptions made, and therefore the viewpoint of the organisation making the analysis. The OEF study has produced a figure of £2.5 billion per annum of additional GDP by 2015 for Heathrow, or £7 billion per annum by 2030 if a third runway is built there. Full implementation of the White Paper runway proposals resulted in a forecast yield of an additional £13 billion per annum in GDP by 2030. Calculations done for the AEF, based on a new runway at Stansted, and which assume increased taxation of the industry, result in a negative economic benefit. Environmental effects External costs, also referred to as hidden costs, are quantifications of the environmental and climate effects of air transport.