How bad is the air quality on the Nipomo Mesa? Spikes in pollution are ‘off the map’

Plumes of dust that waft across the coastal community on the Nipomo Mesa frequently drive air pollution levels above state standards — and that’s happened every year for decades.

Particulate matter pollution in the most affected neighborhood violated state standards 47 days in 2018 and an estimated 42 days in 1999, records show. Some years in between were far worse, with as many as double the number of days in violation.

Those statistics show a measurable problem that has triggered action from local regulators. But the number of violations fails to communicate just how high the levels of air pollution spike during dust events.

The Tribune reviewed public records and shared the findings with multiple air-quality experts, who called the trends unique and outside the scope of air quality standards meant to protect the general population.

Exposure to high levels of particulate matter is a health risk. But there is a gap in current scientific research about the real risk of what people on the Mesa experience: years of repeated exposure to extremely high, short-term spikes.

Patricia Koman is a research investigator in environmental health sciences at the University of Michigan, with an expertise in the health effects of air pollution. She helped form the scientific rationale for current national air-quality standards.

The situation is “off the map of traditional sources of air pollution” and doesn’t fit the national standards, Koman said. Still, it “should require imminent action to protect public health.”

Here’s what the data show:


A Tribune analysis of data from official, regulated air-quality monitors shows frequent and repeated spikes in high concentrations of particulate matter, tiny airborne particles that are unhealthy to breathe. There are elevated levels of both PM 10 — particles smaller than 10 micrometers in diameter — and the much smaller and more dangerous PM 2.5.

Air quality on the Nipomo Mesa is good most of the time, outside of these events that usually last for a few hours at around midday.

Here’s what the spikes looked like two days in a row on a weekend in September.

Air quality on the Mesa often exceeds the state standard for the 24-hour average of PM 10, which is no more than 50 micrograms per cubic meter.

Some days air on the Mesa reaches well above that number, but does not exceed the state standard because of low levels other times during the day.


Spikes of particulate matter are unique to the Nipomo Mesa area in comparison to the rest of the county.

That was the case in mid-September, and most other days. Atascadero sometimes has high PM 10 levels, and pollution is sometimes bad countywide due to nearby wildfires.

Sometimes, PM levels spike multiple days in row, or a few times a month.

High levels of PM 10 concentrations above 250 micrograms per cubic meter of air have been measured dozens of times this year, according to data from the California Air Resources Board.


Extremely high levels of particulate matter spike in other areas of California, too, particularly in the San Joaquin Valley.

Air quality in the Valley is notoriously unhealthy, often without breaks for clean air. Even when PM levels don’t spike, the air is often unhealthy throughout the day in Fresno, for example.

The Tribune pulled data only from monitors that are the same kind as those in San Luis Obispo County, instruments called Beta Attenuation Mass Monitor or BAM.


Air quality on the Nipomo Mesa was discussed in county reports for decades — and violations of state standards have been documented since at least 1989, documents show.

A review of historic annual reports published by the San Luis Obispo County Air Pollution Control District shows that the number of state air-quality violations on the Mesa was higher than those at any other location in the county every year since at least 1999.

PM 10 on the Mesa is estimated to have violated state standards about 42 days in 1999, 90 days in 2000 and 60 days in 2001, county reports say. The worst year since 1999 was 2017, when PM 10 on the Mesa violated standards on 97 days.

It is difficult to compare old reports to current statistics due to changes in how particulate matter is measured.

At that time, air quality was only monitored once every six days. The number of actual violations is estimated by multiplying the number of observed violations by six.

1999 was considered a cleaner-than-usual year and was described as “the lowest occurrence of PM 10 exceedances recorded in our county in the last 10 years.”

All of the spikes recorded in the county that year occurred near the coast on the Nipomo Mesa, it says.


National Air Quality Standards are based on 24-hour averages and annual averages of particulate matter pollution.

But those standards don’t reflect the unique situation on the Mesa, and across the state of California, where levels of particulate matter spike to extremely high levels for short periods of time.

The current national daily PM 2.5 standard is 12 micrograms per cubic meter of air. It’s based on known scientific research the time the standard was reviewed.

Studying the health effects of short-term spikes is challenging and hasn’t been the focus of public health researchers.

As a result, the current research and the standards fail to address the health risk of from that kind of exposure.

Koman said the Mesa is “facing a unique situation, for which the health database isn’t going to support what everyone in the community knows.”

An Independent Particulate Matter Review Panel analyzed the most recent body of medical research on the health effects of PM 2.5 and recommended that the federal government tighten the annual standard to between 8 and 10 micrograms per cubic meter of air to better protect public health.

“Based on full consideration of the overall body of scientific evidence, we unequivocally found that the current standards for fine particulate matter do not protect public health and must be revised,” Christopher Frey, who led the independent panel, told the Union of Concerned Scientists.

[This article was originally published by The Tribune.]