An air pollutant is any gas or particle in the air that causes harm to living things or the environment. Depending on the type of pollutant, the impacts can include respiratory ailments, cancer, birth defects, heart disease, and damage to the environment. Air pollution has these major effects even though it constitutes only a tiny fraction of air molecules. Most air pollutants have a concentration less than one molecule per million air molecules.
There are many types of air pollution. This lesson focuses on the example of tropospheric ozone (also called “low level ozone”) because it can be measured by students, it has been linked to serious respiratory health effects, and because many cities such as Houston exceed federal standards for tropospheric ozone and must find ways to reduce it.
Power plants, factories, vehicles, and other sources do not directly emit ozone into the air. Instead, tropospheric ozone (also called “low level ozone”) forms when emissions of other gases (specifically, nitrogen oxides (NOx) and hydrocarbons (also known as volatile organic compounds, VOCs)) react in the atmosphere in the presence of heat and sunlight. The actual chemical reactions are very complex, but you can think of it in a simplified form as:
Nitrogen Oxides + Hydrocarbons + Heat + Sunlight = Tropospheric Ozone
So to reduce the amount of ozone air pollution that forms in the troposphere, we must find ways to reduce emissions of nitrogen oxides and hydrocarbons from sources like vehicles and industries.
Tropospheric ozone has very different effects than stratospheric ozone. Ozone in the stratosphere protects Earth from the Sun’s ultraviolet rays (see Lesson Four). Even though only about 10% of Earth’s ozone is in the troposphere, it is here that humans and animals can breathe it in as a harmful air pollutant. Tropospheric ozone can have the following health effects, even at concentrations of just 75 parts per billion (i.e. 75 ozone molecules per 1 billion air molecules):
- Make people more sensitive to allergens
- Aggravate asthma
- Damage and inflame the lungs, making it harder to breathe
- Irritate the respiratory system - coughing and irritation in the chest
Children and the elderly are especially sensitive to these health effects. Tropospheric ozone can also interfere with photosynthesis, stunting the growth of some types of plants.
The Houston region has long struggled to meet federal air quality standards for ground-level (tropospheric) ozone. Large amounts of emissions from vehicles and industries (including numerous petrochemical facilities) and hot weather contribute to Houston’s ozone pollution. Though cleaner cars and industries have greatly improved ozone levels in Houston, they continue to exceed federal limits. The U.S. EPA recently lowered the ozone limit from 85 parts per billion to 75 parts per billion, because scientists found that health impacts can occur even at these low levels. This lower limit will make it even more challenging for Houston to reduce emissions sufficiently to attain the ozone standard.
In addition to tropospheric ozone, there are many other types of air pollutants that can cause harm to living things or the environment. Some air pollutants are gases that are virtually invisible. Most occur in very small concentrations (typically less than 1 molecule per million air molecules), but they can still have major impacts on our health and the environment. In Lesson 6, we will see how some of these gases can warm the planet.
Other air pollutants are particles: microscopic liquids or solids like dust or soot suspended in the air. When the air looks “hazy” like in the picture below, it is mostly due to particles scattering and absorbing light. Scientists have learned that high levels of particles in the air can cause cardiovascular and respiratory disease.
The sources of air pollutants are widely varied and include manmade and natural processes. For example, particles are emitted by manmade sources like diesel engines and smokestacks, and by natural sources such as volcanoes and windblown dust. Cows actually produce 80 million metric tons per year of methane, which can warm the planet and help form tropospheric ozone.
The Environmental Protection Agency posts Air Quality Index numbers to tell us how clean the air is in each part of the country each day. More information about this can be found at http://airnow.gov/index.cfm?action=aqibroch.index. In the Houston Chronicle, the weather page shows the ozone levels from the day before and a forecast of today’s ozone levels. Online, the Houston Chronicle has a “current air quality” map that you can click on. That takes you to an interactive map, where they can click on each monitor and see its meteorology and air pollution conditions. For the Westbury schools, the Bayland Park monitor (#13 in the bottom map) would be the closest to their campus. This map is available at http://weather.chron.com/US/TX/Houston.html.
The following AQI ranges correspond to certain ranges of ozone concentration in parts per billion (ppb). This can give students a sense of how their GLOBE ozone measurements (in ppb) relate to AQI levels, though technically the AQI is based on 8-hour averaged concentrations of ozone.
Table 1
|
Air Quality Index
|
Ozone Concentration (ppb)*
|
| 0-50 |
0-59 |
| 51-100 |
60-75 |
| 51-100 |
76-95 |
| 151-200 |
96-115 |
| 201-300 |
116-374 |
| >300 |
>374 |
*These numbers were calculated by using the EPA’s conversion calculator, which can be found at http://www.airnow.gov/index.cfm?action=aqi.aqi_conc_calc.
Answer Key to Lesson 5 Student Worksheets
