Dear Earth Talk: Now that autumn is here, the leaves are going to pile up in my yard again. Is it really that bad to burn them? Why is it illegal to burn leaves in so many places now? — Jeffrey Edwards, Westport, Conn.
Burning fallen leaves used to be standard practice across North America, but most municipalities now ban or discourage the incendiary practice because of the air pollution it causes. The good news is that many towns and cities now offer curbside pickup of leaves and other yard waste, which they then turn into compost for park maintenance or for sale commercially, and there are other burn-free options as well.
Because of the moisture that is usually trapped within leaves, they tend to burn slowly and therefore generate large amounts of airborne particulates — fine bits of dust, soot, and other solid materials. These particulates can reach deep into lung tissue and cause coughing, wheezing, chest pain, shortness of breath, and, sometimes, long-term respiratory problems.
Leaf smoke may also contain hazardous chemicals such as carbon monoxide, which can bind with the hemoglobin in the bloodstream and reduce the amount of oxygen in the blood and lungs. Another noxious chemical commonly present in leaf smoke is benzo(a)pyrene, which has been shown to cause cancer in animals and is believed to be a major factor in lung cancer caused by cigarette smoke. Breathing in leaf smoke can irritate the eyes, noses, and throats of healthy adults, but it can really wreak havoc on small children, the elderly, and people with asthma or other lung or heart diseases.
Sporadic individual leaf fires usually don’t cause any major pollution, but multiple fires in one geographic area can cause concentrations of air pollutants that exceed federal air quality standards. According to the U.S. Environmental Protection Agency, several leaf and yard-waste fires burning simultaneously in a particular locale can cause air pollution rivaling that from factories, motor vehicles, and lawn equipment.
Purdue University consumer horticulture specialist Rosie Lerner says that composting leaves is the most eco-friendly alternative to burning. Dry leaves alone will take a long time to break down, she says, but mixing in green plant materials, such as grass trimmings, will speed up the process. Sources of nitrogen, such as livestock manure or commercial fertilizer, will also help. “Mix the pile occasionally to keep a good supply of air in the compost,” she says, adding that a compost pile should be at least 3 cubic feet and will generate soil conditioner within weeks or a few months, depending on conditions.
Another option is to shred leaves for use as mulch for your lawn or to help protect garden and landscape plants. Lerner suggests adding no more than a 2- to 3-inch layer of leaves around actively growing plants, chopping or shredding the leaves first so they don’t mat down and prevent air from reaching roots.
As for using leaves as mulch for your lawn, it is just a simple matter of mowing right over the leaves with the lawnmower and leaving them there. As with leaves used for garden mulch, this will provide many benefits, including weed suppression, moisture conservation, and moderation of soil temperature.