It is Thursday, October 1st and the air in SF is filled with smoke again today, as it has been, off and on, for weeks. While I am glad not to be in the path of a fire, the smoke pollution is disturbing. Most of news coverage of the pollution concerns its effect on human physical health, which is, of course important. Air pollution causes lung inflammation and reduces our lung capacity. It is particularly dangerous for older people and children. Now, as the coronavirus is on all of our minds, and some of us try to go out less to reduce the chance of infection, the smoke is more disturbing than ever because it reduces our chance for outdoor exercise. For more on human health effects, see Washington Post on air pollution harm to health
While we need cloth masks now, due to the coronavirus emergency, I have learned these do not protect against air pollution. The only protection is an N95 mask or an N95 respirator mask, which most of us do not own.
Gardeners have special concerns. First, we are hesitant to garden in this bad air. I wanted to check apples for ripeness and harvest some that are ripe today, but may not do it as I look out the window and use online sources to confirm what my eyes tell me. Last month I was two days late in watering my community garden. (Fortunately, I didn’t lose any crops, but some of my lettuce was starting to droop.)
I have also had two questions concerning the plants I am growing. The first is what air pollution is doing to them, the second is whether they are safe for me to eat. The answer to the first question is that the effects of air pollution are not good—more on this in a minute. The answer to the second question is one of the only bright spots in my research: pollution does not affect healthfulness of a crop. If the food you harvest has ash or grime on it, wash it off. For good measure you can agitate it in a tablespoon of vinegar in 6 cups of water, then rinse it in plain water. (White vinegar is fine—no need for anything fancy.) You can’t wash off other smog, but plants damaged by it are perfectly safe to eat.
White ash on tomato leaf in late September of 2020. I washed it off to let more light in. (Tomato leaves, which are not edible, are hairy, so they trap more ash than smooth leaves would, or than the smooth-surfaced tomato fruit.)
Back to the effect on plants. Days of smoky air and falling ash will result in a coating on upper plant surfaces. The main effect of this is reduced photosynthesis, since less sunlight can reach the chloroplasts. This will reduce growth of the plant. After a bout of smoky air, it’s a good idea to hose down your garden plants to knock the stuff off of them.
Worse for plants is the more common components of smog, such as oxides of nitrogen or lower atmospheric ozone. (Unlike the ozone layer in the upper atmosphere, ozone near the earth surface is harmful.) These and other polluting gases can enter the chambers in leaves where the plant carries out photosynthesis and interfere with the chemical process. Ozone is particularly harmful. At best, pollution damage will slow the growth of plants. They will make fewer leaves, and will have a smaller root system. This means our food crops won’t produce as much food or as produce it as fast as they would in clean air. At worst, susceptible plants will show symptoms, starting with tiny dead spots on leaves and progressing to dying older leaves. Susceptibility varies among crops and varieties of specific crops. Peas and beans and leafy crops are generally most susceptible, and one study found summer crops more affected than cool-season crops.
(Please note that we are here thinking about pollution of the air, not the soil. If you are worried about soil pollution, you should have it tested for lead and cadmium, and follow the lab’s instructions for what you can grow in it. Also, don’t grow food in a garden that is immediately adjacent to a busy road, especially where vehicles often brake.)
When it comes to air pollution, there is another concern that we should be aware of. When air pollution prevents a plant from photosynthesizing, either by blocking sunlight or interfering with the chemistry of the process, it will reduce the amount of CO2 that plant is removing from the air. This means that pollution-challenged plants are making our excess atmospheric CO2 problem just that much worse. It means that air pollution creates a negative feedback loop, making global climate change happen just that much faster.
Happily, we can safely eat and enjoy any food we succeed in growing in our gardens in polluted air, and are probably not so dependent on our garden that the little bit less production is a problem, but remember agricultural production is also threatened. Our understanding, as gardeners, of what increased air pollution means, not only for humans, but for plants, gives us an early distant warning, and more reason to act, and encourage others to act, to reduce the pollution of our air.
Where and when is air pollution a problem?
Large urban areas across the earth have bad, sometimes chronically very bad air, but it is also a problem in many places we think of as “pristine.” So, for example, Denver has a problem, but so does Rocky Mountain National Park.
One of the worst pollutants is lower atmospheric ozone (O3). It forms when oxides of nitrogen and volatile organic compounds (VOCs) interact in sunlight. The warmer the day, the more ozone forms, so it tends to be worst in afternoons and later in the summer season (just when we are counting on harvesting summer crops).
Where do the components of air pollution come from:
--Well, obviously from fires. And one of the scariest results of recent fires has been the understanding that when modern structures burn, not only is it a horrid loss to the owner, but some of what is released into the air is toxic compounds from the many synthetic materials we use today in building, furniture, and other objects in our homes.
--The biggest other sources are electricity generation (if the plant doing the generating is coal or petroleum-fueled) and gasoline-fueled transportation.
--Agriculture and industrial processes add some to the mix. One study, in Colorado, found that plants making fabrics added significant pollution, because it used petroleum as a fuel and because polyester is a petroleum product.
--Volitile organic compounds (VOCs) come from products like paint or paint thinner, asphalt, oil and gasoline.
What can we do to reduce air pollution:
--Drive less, keep your car well-tuned, keep tires inflated properly, and buy automobiles that burn less gasoline—fuel efficient, hybrid, electric.
--Avoid idling car motors unnecessarily.
--Avoid using other gas-operated motors as much as possible—for example by using an electric lawn mower.
--Avoid using products that add VOCs to the air. Check paint cans and labels of other suspect products for an indication that they are VOC-free.
--Avoid toping off of gasoline tanks when filling them at a gas station. (When you smell gas, it is going into the atmosphere.)
--Do yardwork that might add to pollution in the evening, so pollutants can dissipate before the warm part of the following day.
--Try to reduce the amount of petroleum-source materials you use in your life, for example plastic, polyester in clothing, synthetic fertilizers and pesticides. Eliminate their use if you can, reduce, reuse, and recycle if you do use them.
--Spread the word about what causes air pollution and encourage others to reduce their contributions to it.
Here are two links you can use to learn more:
To check air quality: purpleair.com. Zoom in on specific locations using their world map.
To learn about ozone gardens that assess damage to plants, watch a video on ozone pollution and a lecture by a researcher who studies ozone damage: Ozone Gardens and Ozone Pollution