California air official Mary Nichols, a front-runner to lead the Biden Environmental Protection Agency, is facing liberal criticism that she has ignored the concerns of minority and lower-income people bearing the brunt of toxic air pollution.

Nichols, who is stepping down as chairwoman of the California Air Resources Board this month, has crafted some of the most aggressive climate policies in the country. She has also been a chief opponent of the Trump administration’s environmental regulatory rollbacks, fighting to preserve California’s ability to set its own tailpipe greenhouse gas standards.