Today’s Solutions: September 26, 2023

From The Intelligent Optimist Magazine

Fall/Winter 2016

Air pollution is responsible for roughly 7 million deaths worldwide every year. Air-borne pollutants also cause heart disease, stroke, skin diseases, reduced fertility and miscarriage, and cancer. There’s a solution: Plants that clean the air.
By Emilie Croisier

Walking from the smog-filled air of Delhi—which holds the distinction of having the poorest air quality in the world—into the tranquil Paharpur Business Centre (PBC) is quite literally a breath of fresh air. The first building in India to receive an LEED Platinum Green Building rating, PBC has been touted as “the healthiest building in Delhi.”

A study by the Chittaranjan National Cancer Institute, Kolkata and the Central Pollution Control Board (CPCB), New Delhi, found that workers at PBC had significantly lower rates of eye irritation, respiratory symptoms, headaches, lung impairment and asthma than office workers in other buildings in Delhi. And better air has translated to a better bottom line. According to PBC’s CEO, Kamal Meattle, “Sustainability is good business, and energy efficiency is low-hanging fruit.” As Meattle explained in a 2009 TED talk, the facility’s massive air-cleaning overhaul, inspired by his own deteriorating lungs, has improved productivity by 20% and cut energy costs by 15%.

The secret of Meattle’s success? 1,200 humble houseplants—filling the top floor of the six-story building and spilling out of common spaces throughout the rest. After outdoor air is washed and filtered, these plants use their purifying power to fill this 50,000 square foot building with what the PBC website claims is equivalent in purity to “mountain fresh air.”

This approach is called phytoremediation—the use of plants to remove contamination from air, water and soil. It is a relatively young field capitalizing on a very elegant premise: until we as a species can find a way to stop making environmental messes like air pollution, plants—our evolutionary partners since life began on this planet—may be our greatest ally in cleaning them up. Scientists are only beginning to tap the huge potential of phytoremediation to tackle some of Earth’s biggest environmental and public health challenges. And yet the premise is so simple that it can be easily applied in any home or office with minimal effort, no green thumb required.

The world’s largest environmental health risk

The World Health Organization estimates that air pollution is responsible for roughly 7 million deaths worldwide every year, by far the biggest environmental killer known to man. It’s most closely tied to asthma and other respiratory diseases. But air-borne pollutants have also been implicated in many other conditions including heart disease, stroke, skin diseases, reduced fertility and miscarriage, and cancer.

Outdoor air pollution is only expected to rise in the coming decades, with the Organisation for Economic Co-operation and Development projecting premature deaths due to particulate matter and ground-level ozone to more than double from 2010 levels by 2050. Furthermore, these pollutants eventually fall to the ground and contaminate the soil and groundwater, where they can potentially cause harm for decades to come. For example, even 40 years after the United States started phasing out leaded gasoline, “resuspended” lead from contaminated soil remains a major health concern in urban neighborhoods.

The term “air pollution” tends to evoke images of car exhaust and smokestacks, clouds of smog hanging over urban skylines. But the air pollution inside of our homes can be just as dangerous. Although many outdoor air pollutants like carbon monoxide are generally found at lower levels indoors, the modern home has its own set of toxic air pollutants, especially the volatile organic compounds (VOCs) released by paint, furniture, office equipment and many household products. The best way to get these chemicals out of your home is through good ventilation – leaving a window open. Unfortunately, especially if you live in an urban area, that open window is also the best way to bring outdoor air pollutants in to your home.

As society has struggled to reduce emissions from cars, factories, power plants and consumer products, typically looking toward new technologies to overcome the harm of the old ones, an immediate solution has been staring us in the face—or more accurately, growing up through the cracks in the pavement—all along. But in fact researchers only began to realize plants’ potential to restore our own planet when they envisioned travel to others. 

How to breathe in space

The earliest research on the use of plants to clean indoor air was carried out by NASA in the 1980s, as part of their Clean Air Study. Specifically, NASA was looking for a way to clean the air on space stations, with the goal to develop a closed system that could support long-term human habitation in the vacuum of space. After he left NASA, the lead researcher on the project, B.C. Wolverton, translated these findings into the seminal indoor gardening book, How to Grow Fresh Air, published in 1996.

Meattle’s work in Delhi was directly inspired by the Wolverton’s research. The three plant species he chose for PBC were all included in its original 1989 Clean Air report and selected for their complementary strengths and ease of maintenance. The Areca palm, or “living room plant,” for its high oxygen production during the day; the snake plant or mother-in-law’s tongue, Meattle’s “bedroom plant,” for its high oxygen production at night; and the golden pothos or money plant, the “specialist,” for its superior ability to clear a range of harmful VOCs.

One limitation of NASA’s strategy is the sheer number of plants required for effective air cleaning. The foliage-rich interior of the PBC building isn’t feasible for every space. The Clean Air study suggests having one medium-sized potted plant per 100 square feet of living or office space, and at PBC that ratio is much higher, at 2.4 plants per 100 ft2.

Another limitation is that the Clean Air Study, by design, focused on indoor air—the only air to be found in space. Here on Earth, there’s a whole atmosphere of pollution outside our windows. But many other researchers are now building on the foundation laid by the NASA Clean Air Study and tackling these problems through approaches ranging from practical, “real-world” experimentation to cutting-edge bioengineering.

Improving on perfection

Because the field of phytoremediation is so new, there are many more questions than answers concerning the best plants to use, the best ways to use them, and the best interventions we can make to improve their efficiency.

Researchers at the University of Louisville have partnered with a local school for a “before and after” experiment looking at whether plant barriers can act as a shield against air pollution. It seems likely that they can, but surprisingly this question has never been studied directly before on a single plot of land.

As part of the Louisville Green for Good project, a collaboration among the Diabetes and Obesity Center at the University of Louisville, The Institute for Healthy Air Water and Soil and the City of Louisville’s Office of Sustainability, in October 2016 the lawn of St. Margaret Mary school, which faces a busy road, was planted with a “green buffer”—a mix of shrubs and trees that the scientists and school officials alike hope will improve air quality on school grounds, as well as encouraging civic engagement. As director of the University of Louisville’s Diabetes and Obesity Center Aruni Bhatnagar explained, “One particular outcome we are assured of is that we would pique the interest of the students and get them involved in this as a scientific experiment to develop citizen scientists of the future.”

In Amsterdam, local organizations MyEarth and the AMS Institute are testing the real-world performance of a special strain of honeysuckle called Green Junkie. Green Junkie’s credentials as an exceptional pollution-catcher are partly genetic—the plants were bred to have extra-hairy leaves, stems and flowers to trap particulate matter—and partly a feat of botanical innovation. Along with the plant itself, the team at MyEarth developed a special fertilizer made of plant matter collected from Amsterdam’s streets—and thus already contaminated with air pollutants. This fertilizer somehow induces Green Junkie to grow even longer hairs and, according to Ton van Oostwaard, the founder of MyEarth, to “crave” carbon dioxide.

Back in the laboratory, scientists led by Sharon Doty at the University of Washington have developed genetically modified poplar trees that can remove pollutants from both air and water at far higher rates than their “normal” counterparts. Phytoremediation may be one area where genetically modified organisms can offer a clear advantage over conventional methods in terms of the speed, effectiveness and cost of removing pollutants—especially in critical situations like groundwater contamination. However, it is still not without risk.

One of the reasons Doty and colleagues chose the poplar tree as a candidate for genetic manipulation is that it can grow for several years before producing flowers—theoretically, they can be chopped down before they have a chance to reproduce and potentially spread in the wild. Still, the researchers don’t advocate the large-scale planting of these trees, noting that “[C]ommercial use of these trees requires federal regulatory approval and monitoring, and regulations are becoming increasingly strict for transgenic plants intended for biopharmaceutical or industrial purposes, including phytoremediation. Additional studies are needed to ensure that plant tissues do not cause unacceptable impacts on non-target organisms.”

One day we may be able to walk out of any garden center with a Super Plant that can purify the air in an entire home or office. But until then, we can rest assured that the common houseplants we see all the time already have all the intrinsic properties necessary to continually improve our air quality.   


What is air pollution?

Particulate matter (PM) – Any mixture of solid particles and liquid droplets found in the air, open cooking sources like wood and coal fires are among the worst sources of PM for human health. In developing countries, women are much more likely than men to have respiratory diseases due to their traditional role in food preparation. But PM is also produced in any kind of combustion, from factories to car engines.

Carbon monoxide (CO) – A major pollutant emitted by cars and other motor vehicles, this odorless gas can be fatal in high enough concentrations, because it sticks to the hemoglobin in blood, preventing it from binding oxygen. 

Nitrogen oxides (NOX) – These reactive nitrogen compounds are involved in the generation of acid rain, ozone, and other toxic products.

Sulfur dioxide (SO2) – SO2 is a major byproduct of burning coal, and mainly produced by factories and coal-fired power plants. In the presence of nitrogen oxides, SO2 can be converted to sulfuric acid, the major pollutant in acid rain.

Ozone (O3) – In the upper atmosphere, this highly reactive form of oxygen plays a vital role in blocking the Earth’s surface from potentially harmful ultraviolet light. But when VOCs and NOx react together, they form so-called “ground-level” ozone, which can be very damaging to respiratory health. In one study of 450,000 Americans, those living in cities with the highest ozone levels were 30% more likely to die from lung disease. Ozone is also emitted by printers and photocopiers, contributing to indoor air pollution.

Volatile organic compounds (VOCs) – VOCs are any compound with a very low boiling point, causing them to evaporate into the air. Many are harmless, like vanillin, which gives vanilla its distinct scent. But some, like formaldehyde, are very dangerous—even some we’ve come to find familiar. That “new car smell,” for example? Those are VOCs your body would be better off without. Benzene, found in high levels in car exhaust, is a known carcinogen. Indoor sources of VOCs include paint, varnishes, the protective coatings on electronics and appliances, and cleaning products.

Jerrett et al. N Engl J Med 2009;360(11);1085-95


“Indoor air pollution is much worse than outdoor pollution”

Most people are concerned about cancer. However, heart (cardiovascular) disease is the biggest killer in the world. Heart disease has many causes but in the past twenty years the research has led to a shift in focus: from the individual and his/her diet and (lack of) exercise to the environmental causes. Prof. Dr. Aruni Bhatnagar is researcher with the University of Louisville and the American Heart Association. Bhatnagar coined the term “environmental cardiology”. The Intelligent Optimist spoke with him in Louisville, Kentucky.

The Intelligent Optimist: What is environmental cardiology?

Bhatnagar: “Recent research shows that exposure to pollutants in air, water and soil causes hypertension. There are chemicals in the environment that cause heart disease. That’s why I speak about ‘environmental cardiology’. The Harvard Six Cities Study (1993) found that people who live in more polluted areas die earlier than people who live in less polluted areas. That made sense. More surprising was that eighty percent of those deaths were cardiovascular deaths—not cancer or lung disease (COPD). Today, it’s an accepted fact that people who are exposed to air pollution, are more susceptible to heart disease.”

Pollution causes hypertension; hypertension causes heart disease?

“There are two effects of air pollution. The chronic effect comes through hypertension that leads to lesion formation. Then there is the acute effect. In any city, when air pollution levels go up, six hours later there is an increase in cardiac deaths. Studies with people wearing pacemakers show their pacemaker discharges are higher on the days of more pollution. So there is an acute nervous irritant effect or some vascular effect which precipitates.”

Is pollution the only environmental cause of heart disease?

“No, there’s also the disturbance of the natural environment. When we disrupt the circadian clock of people—because they work night shifts, for instance—you see a direct correlation with an increase in obesity, diabetes and heart disease. Lack of sunlight leads to heart disease too. There is a recent study about the impact of a disease that killed 100 million Ash trees in the United States, in some places fifteen percent of the total tree canopy was lost. In these communities the risk of heart disease substantially increased.”

So pollution and lack of nature kills people. We hear more scientists talking about how global warming may kill millions of people in the future.

“I tell my students that if a volcano erupted and would kill 400,000 people, politics and science would throw all resources to finding out how to prevent such a disaster. Only in the U.S annually a similar amount of people dies from the impact of air pollution, but that seems not to muster much interest. It seems that we have accepted this as a natural part of development. We know what happens to people when a lot of trees around them die; nobody has ever systematically studied what will happen of you deliberately plant trees. That’s research we are now working on.”

Many people think that air pollution is a problem of developing countries?

‘That’s not true. The correlation between pollution and heart disease is not linear. Eighty percent of the risk is imparted at a very low level of pollution. In other words, it’s only marginally better if you live in London instead of in Beijing. It’s like with impact of smoking: If you smoke three cigarettes a day, you have eighty percent of the bad effects people get who smoke two packs a day.”

How can we best protect ourselves in cities from the bad air outside?

“People don’t realize that it gets really bad when you stay indoors. The air pollution levels in homes are much higher than they are outside, especially when the windows are closed. I’m from India but incense is horrible. Same for candles and wood-burning fireplaces. Cooks have the highest rates of heart disease in the world, not because of the food they eat but because of cooking. Couches drenched in toxic flame-retardants. Children clothes treated with formaldehyde to make them wrinkle free.  The list of indoor pollutants is long and nobody knows about that.”

Should these chemicals be banned?

“It makes sense that we don’t want fire in homes. However, it’s interesting to note that the flame retardants were developed by the tobacco companies so that people could smoke indoors. We have to find more natural and environmentally friendly ways to prevent fires. At the same time NASA research shows that certain plants are very effective at removing chemical compounds from the air. According to a recent Harvard study when you put the right plants in offices, there’s an eighty percent increase in cognitive function among the workers.”

What else does recent research say about heart disease prevention?

“You need to walk 30 minutes a day—no need to run. Sitting is the new smoking. Diet is important. There’s no evidence that a high meat diet, even red meat, increases heart disease. What increases heart disease, is processed red meat. You should eliminate all white things from your diet–sugar, white flour, rice, for most people even milk and yoghurt. The only white thing you may eat is cauliflower. In the 1960s and 70s Finland had one the highest rates of heart disease in the world and that was partly because they were having subsidies for pork and for dairy products. When they joined the European Union the Fins could have fresh fruits and vegetables year round. The rates of heart disease plummeted. There was a forty percent change in mortality rates from heart disease within one generation. That’s Finnish miracle and the power of food.”

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