The "redlining" of poor neighbourhoods in the 1930s is associated with higher rates of emergency department visits due to asthma among people of colour today, a new study of eight major California cities published in The Lancet Planetary Health journal has found.

Current rates of emergency room visits due to asthma - a respiratory condition that disproportionately affects people of colour in the United States - are around 2.4 times higher in areas that were "redlined" or deprioritised for mortgage investment in the 1930s, researchers found.

It's well-documented that asthma rates are higher among people of colour, affecting 10.1 percent of non-Hispanic African Americans versus just 8.1 of white non-Hispanics in 2017, according to the US Centers for Disease Control and Prevention.

When it comes to asthma-related deaths, the disparity is even starker: African American adults were almost three times more likely to die from asthma-related causes than white adults in 2014, according to the US Office of Minority Health, and African American children had a death rate ten times that of non-Hispanic white children in 2015.

A lack of access to healthcare along with stress and exposure to air pollution are all factors that have been studied when it comes to the racial disparity in asthma rates, but "what we felt was lacking was a historical view that considered why these factors were concentrated in certain neighbourhoods more than others," said the study's lead author Anthony Nardone, a medical student and researcher at the University of California, Berkeley-University of California San Francisco joint medical programme.

The observational study, conducted by researchers at UC Berkeley, UCSF and Columbia University in New York City, looked at Great Depression-era maps created by the Home Owners' Loan Corporation for eight Californian cities: Fresno, Los Angeles, Oakland, Sacramento, San Diego, San Jose, San Francisco and Stockton.

The maps graded urban neighbourhoods by the perceived risk of mortgage default from A to D (with D representing the highest risk). "Areas that were predominantly inner-city, with black and immigrant residents, were systematically graded as hazardous for investment and outlined in red or 'redlined'. Redlining propelled a lack of investment in those areas perceived as higher risk," the study's authors explained.

"The proportion of the population that was non-Hispanic black and Hispanic, the percentage of the population living in poverty, and diesel exhaust particle emissions all significantly increased as security map risk grade worsened," the study found.

Even 90 years later, those Home Owners' Loan Corporation categorisations seem to still have an effect, the study shows, and "issues around poverty and neighbourhood characteristics really do play an important role" in people's health, said study co-author Neetha Thakur, MD, a pulmonary and critical care physician at UCSF.

"Redlining of these neighbourhoods, coupled with other racist policies at the time, led to divestment of these neighbourhoods," Thakur told Al Jazeera. "Now, it's resulting in redline neighbourhoods being closer to freeways, not having as many resources present in the community, and also decreasing their access to healthcare services."

Living close to a freeway is associated with uncontrolled asthma in children, a 2010 study published in the Journal of Allergy found, with some patients experiencing a twofold increase in risk for uncontrolled asthma if they live less than two miles (three kilometres) from a freeway.

"The decisions that were made around where to build highways in the 40s and 50s likely corresponded to where neighbourhoods were essentially just considered less valuable and where people had less autonomy and advocacy for their needs," Nardone said. "And we know ambient air pollution is pretty strongly tied to asthma."

"What we know about wealth is that it acts as a buffer. It helps people buffer stress; it helps people live in places that might not have exposures to air pollution," Nardone added.

Thakur and Nardone said they hope to look at the bigger picture and see if the association between redlining and asthma rates holds on a national level as well.