Extreme heat and wildfire smoke are independently harmful to the human body, but together their impact on cardiovascular and respiratory systems is more dangerous and affects some communities more than others.

A study published Friday in the journal Science Advances said climate change is increasing the frequency of both hazards, particularly in California. The authors found that the combined harm of extreme heat and inhalation of wildfire smoke increased hospitalizations and disproportionately impacted low-income communities and Latino, Black, Asian and other racially marginalized residents.

The reasons are varied and complicated, according to the authors from the Scripps Institution of Oceanography at the University of California, San Diego and the UCLA Fielding School of Public Health. Structural racism, discriminatory practices, lack of medical insurance, less understanding of the health damages and a higher prevalence of multiple coexisting conditions are among the reasons.