Disclaimer: I used to work for the City of San Antonio (COSA) as an analyst in the Department of Human Services Homeless Division. This post is not going to be about homelessness, but about an email that I would see land in my inbox nearly everyday during the summer months: Ozone Action Day!!!
Ozone Action Days were days when the amount of ground-level ozone was hazardous for everyone’s health, not just for people with respiratory issues. The email provided tips on how to reduce your impact on the environment and not contribute to ground-level ozone accumulation – don’t idle your car, take public transit or walk to work, etc, etc. I’m not sure how many of these emails I saw and I wish I kept track, but anecdotally I’d guess that 55-60% of days between May and October were classified as Ozone Action Days.
It’s been another scorching summer here in San Antonio and as I was walking my dog this morning I thought about the City Health Dashboard site for some reason. I used this site for some school assignments a few years ago, specifically to examine relationships between public health outcomes in Baltimore. Walking in the 7:00am humidity and heat got me thinking about checking out some of the data available on the site for San Antonio.
As someone who believes that public transit, walking, and biking are often the best & most sustainable ways of getting around a city, the negative environmental and health externalities of car-centric planning and culture are often in my thoughts. Tailpipe emissions create a variety of noxious elements, including the above-mentioned ground-level ozone (smog) and particulate matter. Both of these harmful emissions products are known to aggravate asthma, cause bronchial and respiratory problems, and contribute to premature death. High temperatures exacerbate the problems caused by tailpipe emissions by keeping particulate matter and other emissions closer to ground-level. Additionally, the heat generated by cars during operation create higher temperatures in the surrounding area, contributing to an “urban heat island” effect. Tack on homes’ air conditioners having to work much harder to cool a space, and more hot air is being pumped into the atmosphere.
Causes of urban heat islands and pollution are so intertwined and interconnected that books have been written about them, and you (and me) should read them. That’s outside the scope of this post, which makes what I want to do relatively simple. Using data from City Health Dashboard and the 2019 ACS 5-Year Survey, I want to look at average air pollution levels in San Antonio’s 5 poorest and 5 wealthiest census tracts. From there, I can examine other health outcomes in these specific tracts of San Antonio before moving on to the whole of the city. I’m pretty confident in what I’ll find – air pollution is higher in the poorest census tracts of San Antonio than in the wealthier ones, and poor census tracts will also have worse health outcomes. Even if the results are what I expect, I think it’s still valuable to visually compare these metrics and remind ourselves that there’s still a ton of work to be done to create an equitable and healthy city for all residents.
Ozone & Particulate Matter Pollution in San Antonio’s 5 Poorest & Wealthiest Census Tracts
The air pollution data comes from City Health Dashboard and was collected by George Mason University (GMU) using North America Chemical Reanalysis (NARC) – you can read more about the process in the codebook! The data provides monthly average measurements for ozone (ppb) and particulate matter (PM2.5) for 2022 by census tract. Using data from the Census Bureau’s 2019 5-Year Survey, I was able to combine the data to create a single dataset that provides a census tract’s ozone and PM2.5 measurement as well as the tracts’ median household income. From there, I whittled the data down to the 5 wealthiest and poorest tracts by household income.
Below is a table showing the 5 poorest and 5 wealthiest census tracts in San Antonio and their respective median household incomes, along with which zip code each tract is part of. The 78207 zip code makes up most of San Antonio’s West Side, a part of the city historically neglected, marginalized, and segregated from wealthier areas.1 78212, which is just north of downtown San Antonio, contains two census tracts on either ends of the wealth spectrum – 110700 and 190800.
| Census Tract | Part of Zip Code: | Median Household Income |
| 110500 | 78207 | $10,463 |
| 110600 | 78207 | $15,250 |
| 110700 | 78212 | $18,256 |
| 150800 | 78214 | $16,994 |
| 160501 | 78207 | $18,915 |
| 190800 | 78212 | $148,750 |
| 191412 | 78248 | $183,529 |
| 191504 | 78231 | $215,083 |
| 191505 | 78230 | $147,750 |
| 191806 | 78258 | $145,579 |
Below is a map with the above census tracts highlighted over a map of San Antonio (you may need to open it in another window depending on your browser)2. The three census tracts with 78207 are blocks from the downtown district but physically separated by the convergence of two freeways – IH-10 and IH-35. Census tract 110700, part of zip code 78212, is wedged into the southwest corner of the confluence of IH-10 and IH-35. This is a topic for another post, but the built environment, especially the freeway system, of San Antonio and cities across the country can serve as a stout bulwark separating poor areas from wealthier ones.
To graph out the average monthly particulate matter pollution in each of these ten census tracts, I grouped them into “Lowest” and “Highest” Income categories. Throughout every month of 2022, PM2.5 pollution was higher in the poorest areas of San Antonio than in the wealthier areas. While PM2.5 pollution averages actually increased in poorer tracts from February to June, wealthier tracts saw a slight increase followed by PM2.5 pollution improvement over the same period. Although average PM2.5 levels decreased across the city from June to August 2022, PM2.5 pollution still remained higher in poorer census tracts.
Interestingly, ozone levels are typically higher in wealthier income tracts than lower tracts in San Antonio with ozone levels in these tracts mirroring each other throughout the year. Ozone is not emitted directly by tailpipe emissions, making it a secondary pollutant. There could be a variety of factors affecting ozone levels in San Antonio’s wealthier areas, including proximity to high-traffic roadways, such as arterial roads or freeways (though I believe this explanation is unlikely, San Antonio’s extensive freeway system could negate any kind of mitigation methods wealthier neighborhoods employ), or industrial nodes with high emissions. Another potential explanation is that the way GMU collected ozone data from NARC is flawed or that there are more sensors in wealthier areas than in lower-income parts of San Antonio.
Pollution Levels Across Income in San Antonio
We’ve seen how pollution levels vary in the poorest and wealthiest areas of San Antonio. If median income levels are grouped into $30,000 buckets, do we see any difference in pollution levels?
Not really. Census tracts with median household incomes below $60,000 still experience higher PM2.5 pollution than wealthier census tracts throughout 2022. Census tracts with incomes higher than $60,000 still see some high levels of PM2.5 pollution but not nearly at levels experienced by low-income areas of San Antonio. The one outlier in the heatmap below is April 2022, when nearly every census tract experienced high levels of PM2.5 pollution. For the $120-$150K income range, that was the highest PM2.5 levels of the year.
Ozone across the city spreads relatively equally across all income tracts. In fact, we can see slightly higher ozone levels in higher income groups during the summer months than in lower income areas. The winter months of November-January see relatively low levels of ozone.
Other Health Outcomes Across San Antonio’s & Bexar County’s Census Tracts
Using the CDC’s PLACES dataset, we can explore other public health variables in San Antonio & Bexar County. The PLACES dataset contains model-based estimates for a variety of public health outcomes at the census tract level. Joining this data to our City Health Dashboard and Census data by census tract allows to more closely examine some public health variables and their relationship to median income levels throughout the city.
As differences in particulate matter between the 5 wealthiest and 5 poorest census tract were fairly significant, I’m interested in seeing the relationship in other census tracts between particulate matter levels and some other public health outcomes.
Census tracts with median income in the lowest quartile (0-25%) of San Antonio consistently have higher rates of asthma than wealthier census tracts, even when there’s less particulate matter pollution. The same can be seen in high blood pressure rates in San Antonio census tracts. Poorer census tracts have consistently higher rates of residents with high blood pressure than wealthier tracts.
If we only look at public health variables, the health inequality in San Antonio is even starker. There’s a clear positive relationship between diabetes rates and obesity rates in San Antonio’s poor areas. While even some of the richest areas of San Antonio have high obesity rates, none are over 50%. While not as starkly positive a relationship as obesity and diabetes, there’s still a positive relationship in San Antonio between high blood pressure rates and obesity. Again, however, census tracts in the lower quartile experience the worst obesity and high blood pressure rates. Depressingly, coronary heart disease and high blood pressure show a similar positive relationship between them when compared across income groups.
It’s clear that there are massive health disparities in San Antonio when comparing across income groups and I haven’t even gotten into other demographic information, like race, ethnicity or gender, that likely create an even more distressing picture. There’s plenty of reasons why San Antonio’s population looks to be relatively unhealthy – lack of fresh produce in grocery stores, food deserts, car-centric urban sprawl requiring people to spend a lot of time in their vehicles, etc. Without concerted effort, especially in low-income areas of the city, to address these public health deficiencies present throughout the city, San Antonio residents will continue to suffer.
- A good book about the damage done to the West Side of San Antonio, “West Side Rising: How San Antonio’s 1921 Flood Devastated a City and Sparked a Latino Environmental Justice Movement” by Char Miller has been recommended to me by my wife. Don’t worry, it’s on my book list. ↩︎
- Here is a screenshot of the map, as well:
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