In a Life Cycle Impact Assessment (LCIA), essentially two methods are followed: problem-oriented methods (mid points) and damage-oriented methods (end points).
In the problem-oriented approaches, life cycle inventory flows are classified into environmental effects. Mid points covered in most Life Cycle Assessment (LCA) studies are: Greenhouse effect (or climate change), Natural resource depletion, Stratospheric ozone depletion, Acidification, Photochemical ozone creation, Eutrophication, Human toxicity and Aquatic toxicity. These methods aim at simplifying the complexity of hundreds of flows into a few environmental areas of interest. The EDIP or CML 2000 methods are examples of problem-oriented methods. They are generally easier to understand, but can still lead to a considerable number of indicators from which it is difficult to support a decision.
The damage-oriented methods also start by classifying a system's flows into various environmental themes, but model each environmental theme's damage to human health, ecosystem health or damage to resources. For example, acidification - often related to acid rain - may cause damage to ecosystems (e.g., in the Black Forest in Germany), but also to buildings or monuments. In essence, this method aims to answer the question: Why should we worry about climate change or ozone depletion? EcoIndicator 99 is an example of a damage-oriented method. Because there are only 3 types of damages, it is easier to come to a conclusion. The downside is the interpretation/communication of the result (see further).
Problem-oriented methodologies are based on internationally and scientifically accepted approaches when possible. But some categories, such as human toxicity or aquatic toxicity, remain difficult to model and are currently under development and require careful evaluation when used. There are even more difficulties with scientific relevance with damage-oriented methods, hence careful evaluation is necessary.
An important issue with problem-oriented methodologies is the communication aspect of the results. For example, the human health indicator for EcoIndicator 99 uses the concept of "Disability Adjusted Life Years (DALY)." When assessing the life cycle of drinking water production, how do you communicate that producing drinking water constitutes a certain number of Disability Adjusted Life Years?