Life Cycle Assessment (LCA) is a method developed to evaluate the mass balance of inputs and outputs of systems and to organize and convert those inputs and outputs into environmental themes or categories relative to resource use, human health and ecological areas.

The quantification of inputs and outputs of a system is called Life Cycle Inventory (LCI). At this stage, all emissions are reported on a volume or mass basis (e.g., kg of CO2, Kg of cadmium, cubic meter of solid waste). Life Cycle Impact Assessment (LCIA) converts these flows into simpler indicators.

While LCA characterizes emissions and waste over a product's life cycle, it does not allow for a complete assessment of a product's potential impacts, also sometimes referred to as its "safety profile" or its "risk assessment ." This is because LCA reports emissions on a chosen functional unit basis (i.e., 1000 wash cycles or 1 kg finished product).
The exposure and hazard assessments, required as input for the risk assessments, are not part of the LCA. For each type of emission, the probability of adverse impacts can be quantified by risk assessment, taking into account all sources of exposure.
LCA was not designed to do that, but rather it was designed to understand the relative contribution of each stage of the life cycle to certain environmental impact categories. LCA also allows comparisons between equivalent stages of life cycles (i.e., the consumer stage of product A and the consumer stage of product B), provided that the LCIs rely on the same databases and the same assumptions.
Thus, even though LCA cannot tell us whether the use of a product is "safe," it does provide us with "indicators" concerning impact assessment scores of the relative contributions of entire or partial product life cycles to specified impact categories.
As such, it can be a powerful aid for the risk manager in business or in government, which is needed to decide which exposures should be managed first and to communicate this effectively to diverse audiences.
Depending on the goal of the study, the level of detail of an LCA may vary considerably. If it is for internal and screening purposes, the quality of the data may be less scrutinized (or less important) than if the work is going to be used for external claims. For external claims, P&G feels that full compliance with ISO guidelines is a must.

One of the first steps before starting an LCA is to define the "functional unit," which is related to the function that a product or service will deliver. When conducting an LCA on laundry detergent , we often report the results on the basis of 1000 wash cycles. The definition of a functional unit is actually very much linked to the question asked. There is nothing like one functional unit, but many, depending on the type of questions we want to answer. Energy and raw materials consumption as well as associated environmental emissions are calculated on the basis of this functional unit.

To construct a full life cycle of laundry detergent, which involves many different processes, the requirement for data is very important. The range goes from the making of the raw materials, which can take place in different parts of the world, to the making of the detergent product, which takes place in a few, well identified, locations. (P&G has a small number of plants that manufacture its laundry detergent products for the whole of Europe.) Usage and disposal are critical data to collect in order to analyse and understand the life cycle impact of a "product."

Comparing washing systems between countries involves more than just comparing two boxes of detergents. For example, people in Northern Europe do not wash their clothing at the same temperature as people living in the South. Detergent dosage is also different from North to South due to different water hardness.

All of these factors affect the results of a Life Cycle Assessment.