Air Pollution

Objective: to consider the legal definitions of air pollution.

1. air pollution is defined in many ways, but under the U.S. Clean Air Act it is: 
the presence or solid, liquids, or gases
in the outdoor air 
in amounts injurious to humans, animals, plants, or property.
There are three aspects of this legal definition that deserve comment:
air pollutants are not just gases;
despite widespread concerns with indoor air, the legal defiinition of air pollution refers only to outdoors;
and while human health effects are usually the most sensitive, they are not the only effects.  
2.  criteria pollutants, as defined by law, include:
 SOx, NOx, CO, particulates, lead, and ozone.  
These are not only the most prevalent pollutants, but thought to be the best predictors of air quality. 
Our discussion of criteria pollutants will add hydrocarbons because of their contribution 
to ozone and other effects. 
3.  particulates are non-gaseous pollutants suspended in air (also called solid and liquid aerosols)
This is obviously a broad category, and includes the next five definitions:
The first three are all solid materials:
    4.  dust           = solids from various physical processes                           
                              (generally > 1 micron)
    5.  smoke        = solids from incomplete combustion                           
                              (generally < 1 micron)
    6.  fumes        = solids from vapor condensation                       
                              (generally < 1 micron)
The next two refer to liquids suspended in air:
    7.  mist           = liquids from vapor condensation                       
                              (generally < 10 microns)
    8.  sprays       = liquids from atomization of a parent liquid                       
                              (generally > 10 micron)
The reference to the size of these particulates (in microns) will be significant later on 
when we discuss health effects. 
    9.  TSP          = total suspended particulates obtained from 
                             a high-vol sampler of particulates.
TSP is actually an old fashioned, outdated term.  Still, I like to introduce it anyway 
because it helps clarify the history of particulate standards.   Think of a high-vol sampler as a 
fancy vacuum cleaner -- what makes it fancy is that we can accurately measure how much air is sucked 
into the vacuum (measured in cubic meters of air per minute).  We run this vacuum cleaner for 24 hours 
that sucks air through a filter that collects all the liquids and solids suspended in air (i.e., the particulates).
We can then measure two things:  
1. the volume of air sucked into the vacuum over 24 hours (measured in cubic meters);
2. the weight of the collected particulates (measured in grams).  
We can then divide the total grams of particulates by the total cubic meters of air, 
which results in grams per cubic meter.  Since these numbers are usually very small, it is 
often more convenient to express this as micrograms per cubic meter. 
This measure is referred to as the TSP, or total suspended particulates. 
    10. PM-10        = Particulate Matter < 10 microns in diameter.
As I mentioned above, TSP is an outdated term.  The reason for this is that we now know that 
the particulates most associated with health effects are the smaller particulates.  That's because they 
are more efficient at penetrating deep into the lungs (but we'll discuss that in more detail later).  
Since health effects are most associated with particulate matter less than ten microns in diameter, 
standards have been issued for this so-called pm-10.  Incidentally, you can use numbers besides 10 
(e.g., pm-2.5 refers to particulate matter less than 2.5 microns in diameter). 
We obviously have other criteria pollutants to discuss, but this module gets us started. 


Test your knowledge with a: quiz

For more information,  try:   air quality

Return to Home Page  
















Objective: to describe the natural as well as human sources of pollution life cycles.


1. Sulfur oxides:

Also known as SOx, its natural sources include volcanoes, oceans, and general microbial degradation. Human sources are primarily from combustion of fuels containing sulfur. We often refer to crude oils as sweet and sour (sour crude is high in sulfur, while sweet crude is low).

2. Particulates:

Also known as solid and liquid aerosols, particulates are the various non-gaseous pollutants in air. Natural sources include volcanoes, oceans (sea salt), general erosion processes (creating dust), and fires. Human sources are primarily from fuel combustion .

3. Carbon monoxide (CO):

CO is colorless, odorless, and tasteless. Due to these characteristics, CO can be undetected even at dangerous levels. Natural sources of CO are primarily from the oceans -- mostly from the partial oxidation of chlorophylls and methane. Human sources of CO are from incomplete combustion of fuels (from both motor vehicles and industrial processes). Carbon monoxide is usually, by mass, the largest of the criteria pollutants.

4. Hydrocarbons:

Hydrocarbons are in various combustible fuels. Natural sources include bacterial decomposition, and emissions from plants and animals. Human sources are primarily from the oil industry (from incomplete combustion as well as evaporation). Since it is a category (rather than a specific chemical), the source of hydrocarbons depends on the type of hydrocarbon.

5. Nitrogen Oxides:

Also known as NOx, natural emissions of NOx are mostly from microbial action. Human sources are from virtually any combustion process. The heat of combustion combines nitrogen and oxygen (both naturally present in air) by the following equation:

N2 + xO2 ---> NOx (NOx is primarily in the form of NO).

6. Lead:

Lead comes from a wide variety of natural and human sources. Since the development of unleaded gasoline, combustion emissions of lead have dropped dramatically. Nevertheless, total lead sources continue to be a major concern.


Test your knowledge with a: quiz

For more information,  try:   air quality

Return to Home Page  


















Objective: to consider the basic evidence for evaluating health effects.


1. Acute episodes:

Occupational exposures: Because these exposures can be much higher than community levels, they can provide initial evidence of health effects. One major problem with this category is extrapolating this evidence from a population of relatively healthy workers to a general population that includes the very young, the very old, and the very sick. These special populations are usually not found in a worker population, yet they are much more vulnerable.

Acute community episodes: the most notorious examples of acute community episodes include such places as London, Donora (Pennsylvania), and the Meuse Valley (Belgium) in episodes that occurred many years ago. In each of these episodes, sudden and severe changes in pollution (usually from inversions) were followed by sudden and severe changes in mortality rates. These episodes tell us a great deal about higher concentrations, but the challenge is to extrapolate what this means at lower concentrations and chronic exposures.

2. Toxicological studies:

In the previous category, we typically start with an increase in health effects and try to associate that with the higher community concentrations. By contrast, in this category we can start with well defined concentrations, and then test animals for health effects (dose response relationships). Ultimately, we want to extrapolate what this means to humans. There are, of course, various problems with toxicological studies that are best left for other courses to discuss. However, some obvious issues include:

1) the differences between humans and other animals, and

2) most air pollution is chronic (i.e., low concentrations) and mixed (not just one pollutant but many, and they may have synergistic interactions).

More subtle is the issue that most test animals are quite homogeneous, while humans are far more heterogeneous. Therefore, it can be difficult to translate what animal evidence means to the most sensitive humans.

 3. Chronic studies

Much of these principles are covered in your studies of epidemiology and statistics. We generally start with effects and look for relationships with chronic exposures (e.g., in epidemiology, this could be a retrospective study). A statistical approach can use regression analysis to establish associations between tiny increases in exposures and tiny increases in effects. Because the increases and their relationships are not readily apparent, we need the tools of statistics to test these associations.

These studies can be very powerful and useful, but they have their own set of limitations. For example, they are more expensive to conduct, they take more time to establish definitive evidence, and they are subject to many confounding variables.

What have these studies told us? To summarize, tiny increases in pollution are associated with:

1) increased incidence of colds and sore throats; and

2) increased mortality, especially in sensitive persons

(e.g., "COPD" stands for Chronic Obstructive Pulmonary Diseases, which include asthma, bronchitis, emphysema, and lung cancer.)

The most important principle in using these three areas of evidence is that they can be used to corroborate each other. For example, toxicology can tell us that:

  1. particulates smaller than 10 microns (pm-10) penetrate deepest into the lungs.
  2. cilia (tiny hairlike cells that line the resiratory tract) can remove particulates, thereby acting as a defense mechanism.
  3. alveoli (deepest in the lungs) do not have cilia, so the smallest particulates reaching the alveoli take the longest to remove.

Epidemiology can confirm this effect by measure higher incidence of lung disease from smaller particulates.

Thus, if independent toxicological and epidemiological studies both suggest health effects, we have much greater confidence in our conclusions than if we relied on only one approach to evidence. Also, the weaknesses of one approach may be assisted by the strengths of another approach.

Test your knowledge with a: quiz

For more information,  try:   air quality

Return to Home Page  






Objective: to analyze natural and manmade controls of air pollution.

Natural Controls

Just as we considered natural sources of pollution, we should also consider natural sinks :

  1. rain washes out pollutants;
  2. gravity settles out particulates; and
  3. wind can disperse pollutants.

On the other hand, these very same forces can act to accumulate pollutants:

  1. rain can become acid rain;
  2. gravity can increase exposures at ground level; and
  3. wind can trap pollutants horizontally.

Man-made controls

When natural controls are insufficient, manmade controls are needed. It's impossible to give a complete list of air pollution controls within this class, so I offer the following generic classification. These nine generic alternatives can be derived from basic systems theory (general systems theory is always concerned with input, process, output).
















The best way to analyze the above table is with an example. Using cars as the example, gasoline is the input, engines are the process, and exhaust is the output. Strategies for reducing air pollutants are to isolate, treat, or substitute these factors. Therefore, items 1-9 in the table above are defined below with some examples (some of these examples will be described later in the course).

1. isolate the input:

Gas rationing reduces overall emissions by restricting access to gasoline (the input).

2. isolate the process:

PCV valves (described later) retain some emissions in the engine (the process).

3. isolate the output:

National parks have stricter air standards (the output) than most urban areas.

4. treat the input:

Unleaded gas has been treated so as to have no lead.

5. treat the process:

Tune-ups help the engine (the process) run cleaner

6. treat the output:

Catalytic converters (described later) treat the exhaust emissions.

7. substitute the input:

Electric power is a subsitute for gasoline.

8. substitute the process:

Bicycling substitutes human power for the engine.

9. substitute the output:

EGR (described later) recycles exhaust emissions back into the engine.

Which method is the best? It depends, of course, on costs, reliability, and other economic/technological factors. The details for each of these examples is not as important as the idea of thinking systematically about solutions to environmental health problems. That systematic thinking starts with input, process, and output (i.e., systems theory).

Test your knowledge with a: quiz

For more information,  try:   air quality

Return to Home Page  



Gasoline and Engines

Objective: to explain different components of crude oil, and explain their relationship with octane rating, knocking, and engine compression ratio.


The source of gasoline is petroleum (also known as "crude oil" or "fossil fuel"). Petroleum is formed over millions of years from decayed plants and animals. Processing of the crude oil is based on distillation (generally in huge towers), which separate components by their volatility (longer chain hydrocarbons tend to have higher boiling point). These components include (starting with the lowest boiling point for natural gas):

1. natural gas

4. aviation gasoline

7. kerosene

2. liquefied gas

5. auto gasoline

8. fuel oil

3. petroleum ether

6. naphtha

9. lubricating oils

Processing :

Among the above components, auto gasoline obviously carries a huge consumer demand. Therefore, various processing steps can increase the gasoline yield from petroleum. For example:

  1. Cracking refers to the breaking down of long chains.
  2. Polymerization is connecting of smaller chains.
  3. Alkylation builds slightly larger chains and helps increase octane rating (see below).

Output (from engines)

Several terms relate gasoline structure to engine performance:

1. Knocking refers to ignition at wrong time. In general, aromatic hydrocarbons have the best antiknock properties, and straight chain hydrocarbons are worst. However, knocking may be a function of many things, and so we rate anti knocking properties by a measure called octane.

2. Octane is a measure of the tendency to produce knocking. According to a 0-100 scale, n-heptane has a rating of 0, and iso-octane has a rating of 100.

3. Within engines, the compression ratio is a critical measure. By squeezing the air-fuel mixture during combustion, this high compression increases the efficiency of engines but increases knocking. In other words, high compression engines need higher octane fuels.

In the past, we increased octane by adding tetraethyl lead. This technique was relatively cheap but ultimately dangerous because of the health issues surrounding lead. Today, we build octane with such additives as ethanol and MTBE, each with their own set of issues.

Test your knowledge with a: quiz

For more information,  try:   air quality

Return to Home Page  






  1. To identify pollutants from gas stoves and space heaters.
  2. To identify controls for these indoor air pollution exposures.

 1. Gas Stoves

Most American homes rely on gas stoves. By itself, this does not necessarily translate into an indoor air problem. However, a survey in New York City found that half the population used gas stoves for supplemental heating (turning on the stove to warm the house or apartment).This habit translates into increased exposures.

What kind of exposures? Incomplete combustion releases hydrocarbons and carbon monoxide. Furthermore, combustion releases nitrogen oxides. Depending on other conditions, particulate exposures can be significant, too.

Simple dispersion usually reduces the pollutants within an hour, but ventilation techniques are critical. In one experiment, investigators installed a new stove and turned two burners on full for 35 minutes. With a standard ventilation fan above the stove, there was no significant problem. However, when the fan was not turn on, levels reached 2/3 of the federal standards for carbon monoxide. This is assuming no outdoor air pollution.

The lessons are simple: 1) keep equipment in good repair, 2) do not use it in inappropriate ways, and 3) turn on the fan.


2. Home heating

Most American homes use natural gas for heating. Usually, it is centralized heating with external venting, and therefore poses no significant indoor air problem. However, many problems can arise with the use of portable "space heaters." Improperly vented, they can be significant sources of carbon monoxide and nitrogen oxides. Kerosene heaters (normally reserved for outdoor use) can release the same pollutants along with sulfur oxides.

Even the traditional fireplace can be a significant source. Ever curl up by the fireplace and, as time passes, start to feel relaxed and maybe a little sleepy? It could be initial symptoms of carbon monoxide exposure! If it is followed by a headache, the evidence is even stronger.

Of course, I am not suggesting we ban fireplaces or all space heaters. However, simple recognition of sources followed by common sense application of controls (such as ventilation) can be our most important tools in controlling indoor air pollution.


Test your knowledge with a: quiz

For more information,  try:   air quality

Return to Home Page  


Clean Air Act (CAA)

The Clean Air Act was passed in 1963, originally as a guide to states. Federal involvement at that time was limited (remember, the EPA had not been established yet). Major amendments to the act were passed in 1970, 1977, and 1990, giving much more authority to the EPA.

A starting point to understanding the law is to review two philosophies regarding standards: best available technology, and air quality management. Best available technology is essentially focused on emission standards. That is, it centers on the emissions from an individual stack or tailpipe. By contrast, air quality management is focused on ambient standards. That is, it looks at the resulting quality of the actual ambient air that we breathe (i.e., we don't usually breathe directly from the tailpipe!).

Both standards are important, of course, but we have long recognized that since secondary air pollutants are created in the atmosphere, it is not enough to study just the primary emission sources. In other words, we need to understand the entire life cycle of air pollution. Over time, we have had a greatly improved understanding of the influence of individual emission sources on the resulting ambient air quality.

The Clean Air Act mandates the following programs:

State Implementation Plans are required from each state and overseen by EPA. Such plans also recognize air quality control regions (about 247 of them in the U.S., including our own South Coast AQMD in Los Angeles). Each region is classified in one of three ways:

attainment areas, where air quality has been attained, and is subject to no additional requirements;

non-attainment areas: where ambient air standards have not been attained, and therefore are required to take additional actions.

prevention of significant deterioration (PSD), where stricter standards are applied for special areas, divided into three classes (e.g., class 1 is for national parks).

National Ambient Air Quality Standards (NAAQS) address maximum permissable concentrations for criteria pollutants. Two types of standards can be used, each without regard to cost: 1) primary standards, which protects human health; and 2) secondary standards, which protects buildings, crops, water, etc. Notice that the use of "primary" and "secondary" here is different than the way we've used it before.

To assist the public in understanding these standards, the EPA developed the AQI (Air Quality Index), which replaces the older PSI (pollutant standards index ) as a uniform method to measure air quality, and covers five of the criteria pollutants: PM-10, SO2, CO, O3, NO2. Basically a zero means no pollutants and 100 means 100% of the allowable exposure. Anything above 100 would be a violation of the standard.

Three other major programs under the Clean Air Act include:



Test your knowledge with a: quiz

For more information,  try:   air quality

Return to Home Page  




Health Effects of Criteria Air Pollutants

COPD is an acronym for "chronic obstructive pulmonary disease," which includes asthma, bronchitis, emphysema, and lung cancer. Whenever reference is made to "unhealthful for sensitive persons," this includes the very young, the very old, and the very sick. The very sick, in this case, includes individuals with COPD. This is critical to understanding the effects of the criteria pollutants.

Sulfur Oxides are primarily an irritant. They irritate the eyes as well as the lungs. Epidemiologically, they have the strongest association with mortality, especially among high risk populations (the very young, the very old, and the very sick). In other words, SOx aggravates existing health conditions with consequences on mortality rates.

Particulates are also respiratory irritants, and also have a strong association with increased mortality. In addition, they are synergistic with other contaminants, particularly sulfur oxides. Also, we should remember that depending on their chemical identity, particulates may be associated with cancers (e.g., asbestos, berrylium, organics).

Carbon Monoxide is an asphyxiant, meaning that it interferes with the transport of oxygen by hemoglobin in red blood cells. Together, they form carboxyhemoglobin, which cannot carry oxygen. Acute effects are centered in the heart and brain.

Ozone is associated with dryness of throat, and aggravates existing respiratory symptoms. It also decreases resistance to infections. However, ozone is subject to tolerance, which means that after living in an area with ozone, people can be subjected to larger concentrations and not be as affected.

Nitrogen Oxides have similar effects as ozone (you recall that it helps create ozone), but it is roughly 10 times less toxic than ozone. Like ozone, it decreases resistance , which is why associations have been found between photochemical smog and the incidence of respiratory illness.

Lead can be found in a multitude of sources (not just air) and has a long list of health effects. The major effects include anemia, kidney damage, and effects on the central nervous system.


Test your knowledge with a: quiz

For more information,  try:   air quality

Return to Home Page  





Page updated: 02/01/02