Although they are prescribed in legal instruments which stipulate penalties for non-compliance, in practice enforcement has been weak, with firms at times bargaining with local officials over their liabilities. A further problem has been that the levies have been too low to provide an incentive for firms to invest in mitigation efforts. During the 2000s, for example, only after a third increase did the SO2 levy exceed the average cost of pollution mitigation (Schreifels et al, 2012).