QALYs

52

fects are observed a few days later, such as irritation of the respiratory tract, or long-term effects, when effects are observed after long-term exposure and years after the start of exposure, such as chronic bronchitis or lung cancer.

How we said the epidemiology studied the population and the effect of the disease, but we have to take into account that human being has a different way to react. This factor is called susceptible, or individual susceptible individuals, which express a member of population who have the risk to becoming infected by a disease. In our case by the atmo-sphere pollutant. From the studies emerge that the age’s phase has more risk to meet the disease are the child, the elderly, and individual’s whit cardiovascular and respiratory diseases.

Another classification that Epidemiological studies did is the difference between:, obser-vational studies ( they evaluate the distribution of diseases in the population and the determinants of disease) and intervention studies (experimental studies as the exposure of each participant, or each community, is assigned by the researcher with a randomized procedure, which also guarantees the control of other risk factors not known at the time of the study).

53 Observational epidemiological studies are divided into:

• The case-control study starts from the definition of a group of people with the disease of interest and a control group without the disease and compares the pro-portions of exposed to the factor studied in the two groups.

• The cohort study starts from the definition of subjects exposed and not exposed to the factor of interest and follows these groups through time to determine the development of disease in each group.

• The cross-sectional study is based on the observation of the disease and expo-sure to the factor of interest at the same instant in time or in a very short interval.

The studies most used to analyze the effects of air pollution on human health are time-se-ries studies, and ecological studies, which are correlation studies, in which exposure is not attributed to the individual level but is evaluated the variation of data at an average at the group level. These studies add the time lien: the exposure must precede the event and the latter must follow the exposure in a relatively short time. The association between exposure and effect in observational studies is estimated by calculating the relative risk (RR) or probability ratios(Odds Ratio).

The most used studies to analyse the short-term effects of air pollution on human health are time series and case-crossover studies.

The case-crossover design, a particular type of case-control studies, is used to study the existence of an association between short-term exposure and the presence of a specific event that you want to investigate.

These case studies are an association between two phenomena based on a dose-response relationship that must take place following an accepted method, that is by verifying com-pliance with specific causality criteria.

The randomness criteria approved by the scientific community are nine:

• Strength of the association

• Timing sequences

• Dose response relationship

• Specificity of the effect

• Biological plausibility

• Not contradicting the knowledge of other disciplines

• Consistency with other studies

• Analogy

• Reversibility of the effect.

To evaluate the association, or the degree of statistical dependence, between risk factors and the onset of a disease based on observational studies, four categories of subjects can be identified, as represented in the table (fig.3) below.

3. 1.1. Association Measures

SICK NO SICK TOTAL

EXPOSED

NO EXPOSED

TOTAL

A B A + B

C D C + D

A + C B + D A + B ; C + D

Fig. 3. Categories of subjects analyzed to evaluate the association between exposure and disease.

From this scheme it is possible to found the absolute risk (RAs), defined as the ratio be-tween those exposed to the risk factor who became ill and the total of those exposed, it is equal to the incidence of the disease among those exposed:

RAs = a

( a + b )

Another index is the relative risk (RR), equal to the ratio between the incidence in the exposed and the incidence in the unexposed considering the same risk factor:

RR = (RAs exposed) (RAs NO exposed)

(a) (a + b)

(c) (c + d)

=

This ratio is equal to 1 if the factor considered has no influence on the development of the disease; it is higher than 1 when the risk factor plays a role in determining the disease; it is less than 1 if the factor under consideration is protective, i.e. it reduces the probability of contracting the disease.

A further index of association is attributable risk (AR), which represents the share of pa-tients among the exposed that could be avoided if the risk factor were completely remo

Equation 1

Equation 2

55 ved. The RA corresponds to the difference between the incidence in the exposed and the

incidence in the no-exposed:

RA = (RAs exposed) - (RAs NO exposed) = (a) (a + b)

(c) (c + d)

-In addition, a confidence interval can be calculated for the risk estimate, which, with a certain probability, includes the value of the parameter in the general population. The 95%

confidence interval (95% CI) represents the range of values that has a 95% probability of including the true value of the population.

3. 1.2. Relative Risck (RR) associated to pollutants

After this small explanation about epidemiology, we can go more in deep.

From a literature review, we found the cases studies associated on the pollutant of par-ticulate matter. In particular, we will focus on the pollution in PM10, which will be the pollutant of our case study.

In epidemiological studies, we have to do a distinction between the index of mortality and morbidity, to describe the progression and severity of a given health event.

These are useful tools to distinguish the risk factors of diseases and the incidence on the society. As follow there is a definition regarding mortality and morbidity.

Mortality is related to the number of deaths caused by the health event under investi-gation. It can be communicated as a rate or as an absolute number. The mortality index, usually gets represented as a rate per 1000 individuals, also called the death rate. The calculation for this rate is to divide the number of deaths for a period of time for a part of the population by the total population. To keep these values concise and for ease of com-parison to other health events, this number can be multiplied by 1000 to reflect the “per 1000” rate of the target population.[5]

Morbidity, on the other hand, is the state of being symptomatic or unhealthy for a disease or condition. It is usually represented or estimated using prevalence or incidence. Preva-lence describes the proportion of the population with a given symptom.

Equation 3

It is calculated by dividing the number of affected individuals by the total number of indivi-duals within a specific population. It is usually presented as a ratio or as a percentage. [5]

Based on individuals

Based on population group

OBSERVATION STUDIES

O.S.

case-control cohort cross-sectional studies case reports case series

ecological studies time series case-crossover studies

Based on individuals

Based on population group

INTERVENTION STUDIES clinical trial

community trial

I.S.

E.P.

Morbidity

EPIDEMIOLOGY STUDIES

Mortality Morbidity

HALYs

Quality-adjusted life years

Health-Adjusted life years

QALYs