Take a deep breath. In that single intake of air, about 25 sextillion molecules are blown into your lungs, ranging from compounds formed just days ago to billions of years in the past.
In fact, many of the molecules you're breathing in were likely exhaled by members of ancient civilizations and countless humans since.
But what are we all breathing?
About 78 percent of Earth's atmosphere consists of nitrogen produced by volcanic activity beneath the planet's crust. The next major component is oxygen, which makes up 21 percent of Earth's air.
While oxygen molecules have been present as far back as Earth's oceans,
oxygen gas did not appear until ocean-dwelling microorganisms evolved to produce it.
Finally, .93% of our air is argon, a molecule formed by the radioactive decay of potassium in Earth's atmosphere, crust, and core. All of these dry gases make up 99.93 percent of each breath you take.
Depending on when and where you are, the air may also contain some water vapor. But even more variable is the remaining .07%, which contains a world of possibilities.
This tiny piece of air contains numerous tiny particles, including pollen, fungal spores and liquid droplets, as well as trace gases such as methane and carbon dioxide.
The specific cocktail of natural and man-made compounds varies dramatically from place to place.
But no matter where you are, .07% of every breath you take is likely to contain man-made pollutants—including potentially toxic compounds that cause lung disease, cancer And can even damage DNA.
There are a wide variety of known contaminants but they all fall into two categories. First are primary pollutants. These toxic compounds are released directly from man-made or naturally occurring sources.
However, they don't always come from the places you expect. Some large factories produce mostly water vapor, with only small amounts of pollutants mixed in. Conversely, burning wood or dung can produce polycyclic aromatic hydrocarbons.
Dangerous compounds that have been linked to many types of cancer as well as long-term DNA damage. In all cases, pollutants interact with regional weather patterns and topography, which can localize compounds or spread them over kilometers.
As these molecules travel through the air, a change occurs.
Natural compounds called oxidants, formed by oxygen and sunlight, break down pollutants. Sometimes, these reactions wash away pollutants easily with rain.
But in other cases, they result in even more toxic secondary pollution. For example, when factories burn coal, they release large amounts of sulfur oxides.
These molecules oxidize to form sulfates, which combine with water vapor in the air to form a blanket of fine particles that impair visibility and cause severe lung damage.
This so-called sulfur smog was popular in 20th-century London and is engulfing cities such as Beijing. Since the advent of cars, another secondary source of pollution has emerged.
Fossil fuel burning vehicles emit nitrogen oxides and hydrocarbons that react to form ozone.
And while some ozone in the upper atmosphere helps protect us from ultraviolet rays on Earth, this gas can combine with secondary particles to form photochemical smog.
This brown haze covers densely packed cities, making it difficult to see and dangerous to breathe. It also contributes to climate change by trapping heat in the atmosphere.
In recent decades, industrial activities have greatly increased emissions of various trace gases, fundamentally changing the air we all breathe.
Many places have already responded with countermeasures.
Most cars manufactured since the 1980s are equipped with catalytic converters that reduce carbon monoxide and nitrogen oxide emissions.
And today, places like Beijing are fighting smog by electrifying their energy infrastructure and completely limiting automobile emissions.
But while moving away from fossil fuels is important, there is no universal cure for air pollution.
Different regions need to respond with regulations unique to their local pollutants. Because no matter where you live, we all share the same air.
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