During heat waves, trees spew chemicals that worsen air pollution

annual: Adjective for something that happens every year. (in botany) A plant that lives only one year, so it usually has a showy flower and produces many seeds.

chemical reaction: A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

climate: The weather conditions that typically exist in one area, in general, or over a long period.

climate change: Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

combustion: (adj. combustible ) The process of burning.

computer model: A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.

concrete: To be solid and real. (in construction) A simple, two-part building material. One part is made of sand or ground-up bits of rock. The other is made of cement, which hardens and helps bind the grains of material together.

exhaust: (in engineering) The gases and fine particles emitted — often at high speed and/or pressure — by combustion (burning) or by the heating of air. Exhaust gases are usually a form of waste.

filter: (n.) Something that allows some materials to pass through but not others, based on their size or some other feature. (v.) The process of screening some things out on the basis of traits such as size, density, electric charge. (in physics) A screen, plate or layer of a substance that absorbs light or other radiation or selectively prevents the transmission of some of its components.

green: (in chemistry and environmental science) An adjective to describe products and processes that will pose little or no harm to living things or the environment.

HEPA: An acronym for high-efficiency particulate air. It’s a term for a type of pleated air filter (usually made of cloth). It can theoretically remove at least 99.97 percent of airborne particles that are at least 0.3 microns in diameter. That includes most bacteria, dust, mold and pollen. Any particles that are larger or smaller may be trapped even more efficiently. Such filters are often used on vacuums, ventilation systems and as covers to protect the nose and mouth.

hydrocarbon: Any of a range of large molecules containing chemically bound carbon and hydrogen atoms. Crude oil, for example, is a naturally occurring mix of many hydrocarbons.

meteorology: (adj. meteorological) The study of weather as it pertains to future projects or an understanding of long-term trends (climate). People who work in this field are called meteorologists.

model: A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes. Or an individual that is meant to display how something would work in or look on others.

nanoparticle: A small particle with dimensions measured in billionths of a meter.

nitrogen: A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth’s atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn. It comes in two stable forms. Both have 14 protons in the nucleus. But one has 14 neutrons in that nucleus; the other has 15. For that difference, they are known, respectively, as nitrogen-14 and nitrogen-15 (or ¹⁴N and ¹⁵N).

nitrogen oxides: Pollutants made up of nitrogen and oxygen that form when fossil fuels are burned. The scientific symbol for these chemicals is NOx (pronounced “knocks”). The principle ones are nitric oxide (NO) and nitrous oxide (NO 2 ).

organic: (in chemistry) An adjective that indicates something is carbon-containing; also a term that relates to the basic chemicals that make up living organisms. (in agriculture) Farm products grown without the use of non-natural and potentially toxic chemicals, such as pesticides.

oxide: A compound made by combining one or more elements with oxygen. Rust is an oxide; so is water.

ozone: A colorless gas made of molecules that contain three oxygen atoms. It can form high in the atmosphere or at ground level. When it forms at Earth’s surface, ozone is a pollutant that irritates eyes and lungs. It is also a major ingredient of smog.

particle: A minute amount of something.

pollutant: A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.

solar: Having to do with the sun or the radiation it emits. It comes from sol, Latin for sun.

strategy: A thoughtful and clever plan for achieving some difficult or challenging goal.

toxic: Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

vegetation: Leafy, green plants. The term refers to the collective community of plants in some area. Typically these do not include tall trees, but instead plants that are shrub height or shorter.

volatile organic chemicals: VOCs for short. These include a host of solid and liquid chemicals that evaporate (become gases), often at room temperature or lower. Many of these chemicals can be harmful if inhaled or allowed to move through the skin. Concentrations of these chemicals tend to be higher indoors than out. Sources of VOCs include numerous household products, such as paints, varnishes, waxes, oil-dissolving solvents, cleansers, disinfecting, cosmetics, degreasers and glues. Many fuels and green plants also release VOCs.

wave: A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

World Health Organization: An agency of the United Nations, established in 1948, to promote health and to control communicable diseases. It is based in Geneva, Switzerland. The United Nations relies on the WHO for providing international leadership on global health matters. This organization also helps shape the research agenda for health issues and sets standards for pollutants and other things that could pose a risk to health. WHO also regularly reviews data to set policies for maintaining health and a healthy environment.