What if the air were full of dust?

Dust is made of tiny, tiny bits. Some dust is so small it floats in the air.

What if the air were always full of dust? Let's imagine it.

Look at a sunbeam coming through a window and you may see specks of dust floating in it. Outdoors, the air carries dust too, along with smoke, soot and other particles far too small to see.

What if the air were always full of dust? In this thought experiment, you can follow three paths: the giant dust clouds that cross an ocean, the red sunsets that dust can paint, and the ways people keep the air clean enough to breathe.

The EPA uses 'particulate matter' for the mix of solid particles and liquid droplets in air. Some, like dust, soot and smoke, are big or dark enough to see; others can only be detected with an electron microscope.

This thought experiment imagines a sky permanently loaded with particles and asks three questions. How much dust does the real atmosphere already move? What does it do to sunlight and color? And why do air-quality rules focus on the smallest particles? Measurements answer the first parts; the imagined world is reasoning, labeled as ideas.

Strong winds pick up dust in the Sahara Desert in Africa.

The dust flies all the way across the sea to South America. Some of it lands on the rainforest.

The Sahara Desert sends dust across the Atlantic Ocean every year. A NASA satellite measured it: on average, winds carry 182 million tons of dust a year past the west coast of Africa. The dust travels about 1,600 miles over the ocean.

About 27.7 million tons of it falls over the Amazon rainforest in South America. That dust brings phosphorus, a nutrient plants need, from an old dried-up lake bed in Chad. So dust is not only a problem: it also feeds a rainforest.

Using lidar on NASA's CALIPSO satellite from 2007 to 2013, Hongbin Yu and colleagues estimated that an average of 182 million tons of dust a year crosses longitude 15°W and travels about 1,600 miles across the Atlantic. Some settles into the ocean or is washed out by rain; 132 million tons still remain at 35°W, 27.7 million tons fall over the Amazon basin and about 43 million tons reach the Caribbean beyond 75°W.

Some of the dust is picked up from the Bodélé Depression in Chad, an ancient lake bed rich in phosphorus. About 22,000 tons of phosphorus a year reach Amazon soils, roughly what the basin loses to rain and flooding. The yearly amount of dust changed by 86 percent between the dustiest year, 2007, and the least dusty, 2011.

Let's pretend the air is always dusty.

The sky might look hazy. Sunsets might look very red. People might need to stay inside more. This is only pretend.

Now imagine a world where the air was always full of fine dust. Faraway hills might fade into a brownish haze. Sunsets might glow deep red every evening, because dust scatters away blue light.

Breathing might be harder for many people, since the smallest particles can get deep into our lungs. Less sunlight might reach the ground, too. Mars gives us a hint: its dusty sky looks orange in the daytime. These are ideas, worked out from what dust does in our own air.

Extrapolating from real particles gives a reasoned picture. Visibility might fall, as fine particles are already the main cause of haze in parts of the United States, including national parks. Sunsets might redden, since particles scatter blue light out of the direct beam. Health might suffer, because fine particles can reach deep into the lungs and some may enter the bloodstream.

Less sunlight might reach the surface. After the 1991 Pinatubo eruption, sulfate particles in the stratosphere cooled global temperatures by about 0.5°C from 1991 through 1993; that is a measured case, though volcanic sulfate behaves differently from desert dust. All of this is reasoning, not a model run.

Sunlight is made of many colors. Tiny bits in the air bounce the blue light away.

The red and yellow light comes through. That is why a dusty sunset looks red!

Sunlight looks white, but it is a mix of all the colors. Air scatters blue light the most, which is why the sky is blue. At sunset, sunlight travels through much more air to reach you, so even more blue is scattered away and reds and yellows shine through.

NASA explains that small particles of dust, pollution or smoke scatter blue light too, leaving even more red and yellow. That is why sunsets can glow so brightly on dusty or smoky evenings.

Air molecules scatter short, blue wavelengths more strongly than long, red ones, giving a blue sky. Near the horizon, sunlight crosses far more atmosphere, so more blue is removed from the direct beam and the Sun and sky redden. NASA notes that dust, pollution and other aerosols scatter blue light as well, leaving purer reds and yellows.

Particles can also reflect sunlight back to space. USGS explains that after big eruptions, sulfate aerosols in the stratosphere increase this reflection and cool the lower atmosphere; volcanic ash, by contrast, falls out within days to weeks and has little effect on climate.

Tiny bits in the air can be bad for our lungs.

Grown-ups check a color chart that tells how clean the air is. Green means the air is good.

The smallest particles worry scientists most. The EPA says a single hair from your head is about 70 micrometers wide, about 30 times wider than the biggest 'fine' particle. Fine particles can travel deep into our lungs.

To help people know when the air is dusty or smoky, the United States uses the Air Quality Index, a number scale with six colors. Green, from 0 to 50, means good air. The higher the number, the more air pollution and the more people should take care.

The EPA tracks PM10 (inhalable particles generally 10 micrometers and smaller) and PM2.5 (fine particles 2.5 micrometers and smaller). PM2.5 poses the greatest health risk because it can reach deep into the lungs; some may enter the bloodstream. Particles of sand and large dust, bigger than 10 micrometers, are not regulated by the EPA.

The Air Quality Index turns measurements into a number with six color-coded categories. An AQI of 100 generally corresponds to the national short-term standard; values at or below 100 are generally considered satisfactory, and values over 300 are hazardous. Air-quality alerts let people reduce their exposure on bad days.