Croatian air pollution levels reflect the drop in industrial production recorded in 1991 at the onset of the Croatian War of Independence—pre-war emission levels were only reached in 1997. The use of Hydrodesulfurization fuels has led to a 25% reduction of sulphur dioxide emissions between 1997 and 2004, and a further 7.2% drop by 2007. The rise in NOx emissions halted in 2007 and reversed in 2008. The use of unleaded petrol reduced emissions of lead into the atmosphere by 91.5% between 1997 and 2004. Air quality measurements indicate that the air in rural areas is essentially clean, and in urban centres it generally complies with legal requirements. The most significant sources of greenhouse gas (GHG) emissions in Croatia are energy production (72%), industry (13%) and agriculture (11%). The average annual increase of GHG emissions is 3%, remaining within the Kyoto Protocol limits. Between 1990 and 2007, the use of Ozone depletion was reduced by 92%; their use is expected to be abolished by 2015.
Question: How many years did it take for the reduction of sulphur dioxide emissions to drop by 25%?
Answer: 7