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Surprising Sources of Air Pollution

Air pollution is an insidious problem that doesn't recognize borders and travels thousands of miles. In fact, Americans are producing less pollution but experiencing greater amounts of smog on the West Coast, the result of pollutants originating in Asian countries.1 In 1991, the U.S. and Canada entered into an agreement to address transboundary air pollution that led to a reduction in the production of acid rain.2

A collaboration of more than 40 researchers looking at data from 130 countries has called air pollution the "largest environmental cause of disease and premature death in the world today."3 Nine million premature deaths were attributed to air pollution in 2015 — 16 percent of all deaths worldwide and three times more than AIDS, tuberculosis and malaria combined. Fine particulate matter is the most studied type of air pollution and refers to particles smaller than 2.5 micrometers in diameter.

Particles this size are small enough to pass through your lung tissue and enter your bloodstream, triggering chronic inflammation and chronic disease. According to the World Health Organization (WHO), 92 percent of the world's population is breathing polluted air.4 Air quality measurements were based on outdoor sources from transportation vehicles, industrial activity, coal powered plants and burning household fuel.

However, while the numbers are considerable, they may be conservative as WHO did not factor ozone or nitrogen oxides, which are also known air pollutants.5 Emissions of nitrogen oxides combine with oxygen and sunlight to break down into ozone. Levels of this air pollutant has tripled since 1990.6 Scientists have identified a surprising source of nitrogen oxides pollution that contributes a greater number of fine particle pollution than previously anticipated.

High Levels of Nitrogen Oxide Not From Car Exhaust

California has the strictest car emission standards in the U.S. but continues to be plagued by nitrogen oxides pollution. Although pollution levels are declining, they are not going down as quickly as expected. The reason? Researchers now find nitrogen oxides are being released into the atmosphere from synthetic nitrogen-based fertilizers.7

California's vast agricultural lands may be responsible for as much as 51 percent of nitrogen oxides off-gassing across the state, especially in areas that use synthetic nitrogen-based fertilizers.8 Nitrogen oxides is a catchall term used to designate nitric oxide and nitrogen dioxide, both of which react with oxygen and sunlight to produce ozone in lower atmospheric levels.

This can trigger respiratory conditions in children to the elderly, including asthma and emphysema. Maya Almaraz, Ph.D., researcher at the University of California-Davis who led the new study on California farmland, said, "The potential impact this could have on health, especially in rural areas, is definitely on our radar."9 First running a mathematical model, Almaraz determined farmland was emitting 52 percent more nitrogen oxides that previously anticipated.

After seeing another study that demonstrated nitrogen oxides may be emitted from the soil,10 Almaraz's team began modeling soil emissions in different areas of the state and then double-checked their findings by comparing them to measurements made by other scientists.11 These high levels of nitrogen oxides may have 300 times the heat-trapping power of carbon dioxide.12

Measured Amounts Higher Than Anticipated

Researchers have known soil microbes convert nitrogen-based fertilizers to nitrogen oxides and release them into the air. However, it was estimated that only 1 kilogram of gas was produced per 100 kilograms of fertilizer, or roughly 1 percent. Researchers thought the amount of gas would increase linearly, or stay at 1 percent of the amount of fertilizer used.13 These predictions turned out to be conservative, as emissions were measured at up to 5 percent of the fertilizer used.

Further experimentation found the increase was exponential and not linear, as the original research didn't account for conversion when excess nitrogen fertilizer was applied to the fields.14 To determine if these estimates held up globally, biochemist Philip Robertson, Ph.D., and his team evaluated emissions across 84 worldwide locations, confirming the exponential boost when excess fertilizer is applied.15

This high rate of nitrogen oxides emission is a large departure from what the state of California Air Resources Board had previously assumed was 3.8 percent pollution coming from farmland soil.16 Researchers measured the highest levels of nitrogen oxides pollution in California's most highly-fertilized Central Valley.

As noted by Benjamin Houlton, professor at UC Davis' department of land, air and water resources and one of the authors on the paper,17 six of the U.S. districts with the worst air quality were included in the area studied.

In California, children living in the Central Valley have the highest rate of asthma in the state,18 and the San Joaquin Valley within the Central Valley has the highest rate of asthma in the U.S. California is already poised to mobilize resources to address the problem as groundwater issues associated with synthetic, nitrogen-based fertilizers were identified several years ago.19

Scientists alerted the state that excess fertilizer has been leaching into groundwater20 and contaminating drinking water, and that runoff is triggering toxic algae blooms,21 severely damaging the aqua environment.

Synthetic Fertilizers Do Damage Below the Surface

Damage to the environment from nitrogen-based fertilizers includes air, water and soil pollution. When added to the soil, these synthetic fertilizers severely disrupt the soil microbiome, necessary for establishing and supporting strong, healthy plant life.22 Researchers from the University of Toronto have published data that support previous research demonstrating microorganisms living in the soil are critical to the growth and health of plants.

The soil microbiome acts as an interface between the plant and the soil; it plays a significant role in nutrient uptake from the soil, and signals plant development.23 Researchers used 30 species of plants grown in identical soil mixtures in a controlled laboratory setting. The plants were raised for 16 weeks, exposed to permissive and simulated drought conditions.

The researchers looked at the diversity of the root microbiome across the species, finding related plants had more similarity between root microbiomes than diverse species. Lead author, Connor Fitzpatrick, University of Toronto, commented on the results, saying:24

"It's as you would expect. Just as there are more similarities between a human's gut microbiome and an ape's than between a human's and a mouse's, the closer the relationship between plant species, the more similar their root microbiomes. It's important to document as a way to better understand the evolutionary processes shaping the plant root microbiome."

As you might expect, when synthetic fertilizer is added to the soil it adversely affects the soil microbiome,25 reducing the diversity and affecting plant growth. The net effect of synthetic, nitrogen-based fertilizers is to reduce the soil's organic matter26 and its long-term ability to store organic nitrogen. Research comparing two crop seasons found the addition of nitrogen-based fertilizer also reduces the pH and decreases bacterial diversity in the soil.27

Modern farming practices rely on monocropping, planting the same crop each year on the same land. Further research has demonstrated the use of organic or bioorganic fertilizer treatments maintains yield during monocropping, while protecting soil diversity.

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