Question

Why might it be difficult to establish a direct link between air pollution and health problems?

Answer 1

It is difficult to draw a cause-effect relationship in a specific instance of a disease state.

Explanation:

That was kind of convoluted lets discuss this a bit more.

So, lets say a single child living in Pollution-ville (population 30,000) was just recently diagnosed with asthma. Pollution-ville has numerous factories, most of these factories release sulfur dioxide (`SO_2`) a known toxic gas. Unfortunately, despite the fact that `SO_2` is a toxic gas and may have been a major etiological (causative factor) in the child's asthma diagnosis we can't attribute exposure to `SO_2` since there are many other factors that could have been the cause of the child's asthma (genetics or infections).

This is where epidemiology comes into play. Epidemiology is the study of patterns, causes, and effects of health and disease in a population.

Lets say that this child's asthma diagnosis is the 60th newly diagnosed case of pediatric asthma in the past month. All of the newly diagnosed children live in Pollution-ville. This large number of new diagnoses point toward a common environmental factor rather than a random event.

Lets continue in our case against `SO_2`. A town named Bad Aire, is 1,000 miles (1,609 kilometers) to the east of Pollution-ville and has the same problem of `SO_2` releasing factories and numerous newly diagnosed cases of pediatric asthma. An epidemiologist may see this trend between the two cities and hypothesize that these trends are correlated to the release of `SO_2`.

The epidemiologist would then look at other cities (besides Pollution-ville and Bad Aire) with factories that release `SO_2` and the number of newly diagnosed pediatric asthma cases. Numbers and trends however can only strengthen the hypothesis. In order to prove definitively that `SO_2` released from factories can be a causative agent experiments must be run in a lab using pure `SO_2` in animal models.

In the most general case we would need to find a general trend which would need to be tested in a lab to prove that the particular agent is linked to a disease state. We should also keep in mind that most factories and other sources of air pollution release more than 1 chemical into the air and any one of these chemicals could be our disease causing agent.

This makes it can be incredibly difficult to establish that a particular pollutant is responsible for a particular disease state.