The 1990 Amendments also required reductions in nitrogen oxide (NOx) emissions at Phase I units. The key factors in NOx formation are flame temperature and oxygen levels present for combustion.[10] Installation of low-NOx burner retrofits are the most common means of compliance, generally reducing emissions from uncontrolled levels by up to 50%.[11] Many utilities complied with requirements by installing stack-gas scrubbers and low-NOx burners at the same time. Low-NOx burner technology was readily available, and considerably less expensive than installation of scrubbers,[12] so control of NOx was considered less demanding by most electric utilities.