Protection from Cumulative Emissions and Underenforcement of Environmental Law Act of 2021 This bill directs the Environmental Protection Agency (EPA) to address the cumulative public health risks associated with multiple environmental stressors and recommend measures to reduce the number of violations of environmental law in certain environmental justice communities. Environmental justice communities are communities with significant representation of communities of color, low-income communities, or tribal and indigenous communities that experience, or are at risk of experiencing, higher or more adverse human health or environmental effects, as compared to other communities. In addition, the EPA must publish and implement a proposal for a protocol that assesses and addresses the cumulative public health risks associated with multiple environmental stressors, such as impacts associated with global climate change. Further, the EPA must identify at least 100 communities that (1) are environmental justice communities; and (2) have had more environmental law violations than the national average, as determined by the EPA. The EPA must then identify the causes of the violations, identify measures to reduce the number of violations, and implement such measures.