In numerical terms, ground-level ozone, alone, has been estimated to cause 10% to 40% growth loss, premature aging, and a decrease in pollen lifespan resulting in an estimable cost in agriculture of $2 billion to $6 billion per year (Nebel and Wright 533; "Smog"). Losses in crop yields were estimated to be 20% to 30% between 1989 and 1992 (Nebel and Wright 533). In Ontario alone, smog was attributed to reduce crop yields equivalent to $70 million per year ("Smog"). In a study conducted by the EPA to Congress, continuous implementation of a Clean Air Act Amendments over the period 1990 to 2010 would accumulate a minimum 1999 net present value of agricultural benefits of at least $4 billion (EPA, "The Benefits and Costs" F-8). Along with the fact that 60% of the world's food is produced in countries that also produce 60% of the world's air pollution, the significance of clean air is clearly seen (Nebel and Wright 533).