By Dr. Mercola

Outdoor air pollution is a serious environmental health risk linked to both chronic and acute health conditions, including stroke, heart disease, lung cancer, asthma, chronic obstructive pulmonary disease, and respiratory infections.

According to the World Health Organization (WHO), ambient (outdoor) air pollution in both cities and rural areas caused an estimated 3.7 million premature deaths worldwide in 2012, the majority of which were due to heart disease and strokes.¹

As WHO noted:²

“Most sources of outdoor air pollution are well beyond the control of individuals and demand action by cities, as well as national and international policymakers in sector like transport, energy waste management, buildings and agriculture.”

This is largely true, but there is one environmental change that could have a dramatic influence on air pollution, and its effects on human health, independent of these other factors, and that is planting more trees (especially in urban areas).

A new study actually quantified the benefits to human health from trees’ impacts on outdoor air pollution, and they were quite remarkable.

Trees Save Close to 1,000 Lives, and Billions in Health Costs, Each Year

In the first broad-scale estimate of air pollution removal by US trees nationwide, researchers found that trees and forests in the US removed 17.4 million tons of air pollution in 2010, with human health effects valued at $6.8 billion.³

Although this pollution removal equated to an average air quality improvement of less than 1 percent, its effects on human health were significant, especially in urban areas. The health impacts included the avoidance of more than:

• 850 deaths
• 670,000 cases of acute respiratory symptoms
• 430,000 incidences of asthma exacerbation
• 200,000 school days lost

As you might suspect, most of the pollution removal occurred in rural areas (where tree cover can be as high as 88 percent) but most of the health impacts were within urban areas (where air pollution tends to be worse and population levels are higher).

Previous research has also shown that pollution removal by trees impacts human health. One study found that a 10-by-10 kilometer space (approximately 6-by-6 miles) with 25 percent tree cover in London could remove more than 90 tons of particulate matter annually, which would lead to the avoidance of two deaths and two hospital admissions per year.⁴

Trees remove air pollution primarily by uptake of pollutants via leaf stomata (pores on the outer “skin” layers of the leaf). Some gaseous pollutants are also removed via the plant surface.

Once inside the leaf, the gases “diffuse into intercellular spaces and may be absorbed by water films to form acids or react with inner-leaf surfaces.” According to the researchers:

“Trees affect air quality through the direct removal of air pollutants, altering local microclimates and building energy use, and through the emission of volatile organic compounds (VOCs), which can contribute to… [air pollution] formation.”

Adding trees to urban areas was deemed to be particularly important, given the trees’ close proximity to people. The researchers added:

“…96.3 percent of pollution removal from trees occurred on rural land. However, as human populations are concentrated in urban areas, the health effects and values derived from pollution removal are concentrated in urban areas with 68.1 percent of the $6.8 billion value occurring with urban lands.

Thus in terms of impacts on human health, trees in urban areas are substantially more important than rural trees due to their proximity to people. The greatest monetary values are derived in areas with the greatest population density (e.g., Manhattan).”

‘Living Air Purifiers’: Harnessing the Power of Plants to Improve Your Air Indoors

Most people spend as much as 90 percent of their time indoors, where indoor air quality can be up to five times worse than outdoor air, which can have a very detrimental impact on your health. For example, according to the US Environmental Protection Agency (EPA), poor indoor air quality can cause or exacerbate:

• Asthma, allergies, and other respiratory problems
• Headaches
• Eye and skin irritations
• Sore throat, colds, and flu
• Memory loss, dizziness, fatigue, and depression

Long-term effects from exposure to toxic airborne particles include heart disease, respiratory disease, reproductive disorders, sterility, and even cancer. I’ve previously discussed how to lower your health risks from air pollution in detail, but one way to do so is to add some houseplants to your home. Similar to trees outdoors, indoor houseplants can naturally remove toxins from your home’s (or office) air.

It was NASA, along with the Associated Landscape Contractors of America (ALCA), that conducted the classic study on the benefits of plants on indoor air, and they reported that houseplants were able to remove up to 87 percent of air toxins in 24 hours.

They recommended using 15 to 18 "good-sized" houseplants in 6- to 8-inch diameter containers for a 1,800-square-foot house. NASA at Stennis Space Center has also constructed a BioHome that uses bioregenerative technology with the ultimate goal of providing a life support system for permanent human habitation of space.

And inside the structure are common houseplants, which NASA says “serve as living air purifiers” to “absorb chemical pollutants resulting from synthetic materials in the living area.”⁵ If houseplants are capable of cleansing air in the BioHome, imagine what they can do in your home! According to The New Ecologist, the top 10 anti-pollutant houseplants are:⁶

The Feston Rose plant	Devil’s Ivy
Phalaenopsis	English Ivy
Parlor Ivy	African Violets
Christmas Cactus	Yellow Goddess
Garlic Vine	Peace Lily

Nature Is Teeming With Health Benefits

The fact that trees help to absorb and mediate some of the chemicals humans add into the environment is but one example of our intricate ties with nature, and of nature’s power to impact health and healing. Human DNA actually contains much of the same material found in the plant world, so perhaps that’s why living closer to nature can help you to live longer, and hospital patients who have a view of nature recover from illness and surgery more quickly than those who don't.

Most synthetic medications are based on mimicking the action of compounds found in plants. Scientists cannot create these substances but must, rather, try to make copies. But in their synthetic models they often end up with compounds that your body doesn’t recognize and doesn’t know how to handle.

A plant, however, is a complex of thousands of biomolecules, many of which are countervailing, so if there’s one effective compound that may have a toxic effect, it usually contains a countervailing compound so that it doesn’t harm your liver, for example. It’s the interplay of chemicals that make the plant work, which is why you can’t study herbal medicine by isolating a certain element; you’ve got to study the whole plant. (This is what conventional medicine is largely missing.)

When I interviewed Donnie Yance, a clinical master herbalist, he explained that foods and herbs share quite a few similarities, including being pleiotropic -- which means they produce more than one effect. Herbs can help support your health from a very basic level, just as foods do. Of course, the ultimate “herbalism” is the food that you eat on a daily basis. Dark green leafy vegetables, herbs, and spices are excellent sources of antioxidants, anti-inflammatories, and anti-cancer substances that can dramatically influence your health. All of this is part of our intrinsic connection to nature.

This connection continues when you use plants for healing, including when you prepare tinctures or teas from herbs, which you can do in your own kitchen. It continues in your garden, when you grow food or plant a flower, and via trees, whether a new sapling is planted in an urban area or you tend to a decades-old tree in your backyard. According to many herbal experts, this relationship with plants and nature is nearly as important as the herbal medicine itself.

Do You Want to Plant Trees in Your Backyard?

Every tree planted helps the environment, and trees around your home can increase your property value by more than 15 percent and improve your odds of a sale. Trees also do the following wonderful things for you and the environment:

• Decrease carbon dioxide and increase oxygen levels in the atmosphere
• Improve water quality and reduce erosion
• Give songbirds a home, and provide food for all kinds of wildlife
• Provide shade in summer and a windbreak in winter, thereby reducing your cooling and heating costs
• Beautifying your home and neighborhood, and adding curb appeal

If you want to plant some trees but are not sure how to go about it, organic arborist and author Howard Garrett (aka, the Dirt Doctor) can end your ambivalence with his simple, straightforward steps to tree planting.⁷ Planting a tree the right way involves six basic steps:

1. Dig a wide rough-sided hole
2. Run a "perk test" for drainage
3. Prepare the root ball
4. Set the root ball in the hole with backfilled soil
5. Settle the soil with water
6. Mulch the surface

According to Garrett, almost all trees planted today are being planted incorrectly. The most serious problem is that they are planted too deep. When the top of the root ball and the root flare are buried under the ground, hidden roots can circle and "girdle" the trunk, choking off nutrients and weakening the tree, which makes it susceptible to blowing over. Another problem is, when soil comes up too high on a trunk, the covered bark tissue stays moist all the time and plant growth is dramatically slowed or even stopped. The health of your soil is also of crucial importance.

One of the most important steps for the long-term success of your tree would be to create as large a ring as possible around the tree, a minimum of three feet but as large as 10 feet, and cover the area with wood chips. If there is grass, there is no need to remove it, merely lay cardboard over the grass and pour the chips over the cardboard. Ideally, put 12-24 inches of wood chips. It is okay to put the chips next to the tree; they won’t damage the trunk. You typically can get the chips for free from a local tree cutter. The chips are an earthworm magnet and one of the best additions to increase soil fungi.

Plants have a highly complex underground communication network, formed by a type of fungi called mycorrhizae. Mycorrhizae attach to the roots of plants, sending out fine thread-like filaments to the roots of other plants and forming an underground web that can stretch dozens of meters in a virtual “plant Internet.”

These filaments not only increase nutrient uptake 100 to 1,000 times, but also serve as an early warning system to connected plants so they can build up their defenses when a threat presents itself. But in order for these networks to exist, the soil must be undisturbed. Erosion, tillage, cultivation, compaction, and other human activities destroy these beneficial fungi, and they are slow to colonize once disrupted. Therefore, intensively farmed plants don’t develop mycorrhizae and are typically less healthy as a result.

Can Outdoor Air Pollution Be Avoided?

If you happen to live in a heavily polluted area, the best option is to move, but I realize that isn’t always a practical option. If you can’t move, pay attention to the Air Quality Index (AQI), released by the EPA to calculate five major air pollutants:

• Ground-level ozone
• Particulate matter
• Carbon monoxide
• Sulfur dioxide
• Nitrogen dioxide

If the AQI in your area is high, it may be best to stay indoors as much as possible. At the very least, avoid exercising outdoors when air pollutants are high (such as during rush-hour traffic). The truth is, however, that you can’t always escape outdoor air pollution, so it’s better to focus your attention on your immediate environment, which you have more, if not full, control over. The most effective way to improve your indoor air quality is to control or eliminate as many sources of pollution as you can first, before using any type of air purifier.

This includes accounting for molds, tobacco smoke, volatile organic compounds from paints, aerosol sprays, and household cleaners, pesticides, phthalates from vinyl flooring and personal care products, pollutants from pressure-treated wood products, radon gas, and more. The next step to take is free—simply open some windows. Of course, this can only take you so far, and works better if your outdoor air isn’t heavily polluted, but it's an important and simple step.

Next, since it is impossible to eliminate all air contaminants, one of the best things you can do is incorporate a high-quality air purifier. My recommendations for air purifiers have changed over the years, along with the changing technologies and newly emerging research. There are so many varieties of contaminants generated by today's toxic world that air purification manufacturers are in a constant race to keep up with them, so it pays to do your homework. At present, and after much careful review and study, I believe air purifiers using Photo Catalytic Oxidation (PCO) seem to be the best technology available.