HEALTH EFFECTS FROM AIR POLLUTANTS
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:
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.
For more information, try: air quality
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 :
- rain washes out pollutants;
- gravity settles out particulates; and
- wind can disperse pollutants.
On the other hand, these very same forces can act to accumulate pollutants:
- rain can become acid rain;
- gravity can increase exposures at ground level; and
- 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).
input
process
output
isolation
1 2 3 treatment
4 5 6 substitution
7 8 9 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).
For more information, try: air quality
Objective: to explain different components of crude oil, and explain their relationship with octane rating, knocking, and engine compression ratio.
Source:
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:
- Cracking refers to the breaking down of long chains.
- Polymerization is connecting of smaller chains.
- 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
Back to HOME PAGE