Interpretation of metrics: DALYs and damage to human health
One reason why we care about the environmental impact of our choices is that they affect human health. But how do you quantify that? The most commonly used metric is Disability-Adjusted Life Years (DALY), taking into account the years lost to premature death and expressing the reduced quality of life due to illness in years as well. How can you use this in life cycle assessment and what are the points of contention you should be aware of?
The concept of the DALY was introduced to life cycle assessment by Patrick Hofstetter in the mid-nineties when he was looking for a uniform metric to express human health damage. The DALY is an established term in the medical world and has been adopted by many well-known endpoint impact assessment methods like ReCiPe and Impact2002+. There, it is used to quantify the burden of human disease resulting from environmental pollution and attribute it to the life cycle of products or services.
How do DALYs reflect damage to human health?
DALYs measure the gap between an ideal situation in which everyone lives to the standard life expectancy in perfect health and the actual situation. The metric is a combination of the years of life lost (YLL) due to premature mortality and the years of life lost due to disability (YLD) when living with the disease or its consequences: DALY = YLD + YLL (See the image at the top of the page. Image source: Wikimedia Commons.).
Multiple causes for human health damage
Environmental pollution can lead to human health damage in many different ways. Some examples:
- Direct effects of climate change: heat waves, air pollution and aeroallergens
- Indirect effects of climate change: malnutrition, spread of water-borne or vector-borne infectious diseases
- Ozone layer depletion: increased risk of skin cancer and cataracts
- Exposure to toxic chemicals: various diseases including cancer
- Exposure to ionising radiation: cancer and hereditary effects.
- Photochemical oxidants such as ozone: increased frequency and severity of respiratory diseases such as asthma and COPD
- Exposure to particulate matter: respiratory health damage
Objective and subjective basis
Whereas YLL can be derived from objective statistics, YLD is more subjective. A disease’s disability weight is expressed on a scale from 0 (perfect health) to 1 (death). For instance, a disability weight of 0.043 for asthma implies that the burden of living with asthma for one year is equivalent to losing approximately 16 days due to premature mortality. For dengue haemorrhagic fever, the average disability weight is 0.545, which is equivalent to losing more than 28 weeks per year due to premature mortality. The list of disability weighting factors used by the World Health Organization (WHO) can be found here.
DALY is subject to social preferences
Impact assessment methods are full of normative choices, and so is the DALY. Should lost years of healthy life be valued more at some ages than at others? Some people think so, and use age weighting to take this into account. The WHO often applies non-uniform age weighting for their Global Burden of Disease (GBD) studies, which means that they value a year of life in young adulthood more than a year in old age or in infancy. But not everyone agrees that younger and older ages should be given less weight. Furthermore, there is a debate about how big the weighting factor should be.
Another point of contention is whether a year of healthy life now is worth more to society than a year of healthy life sometime in the future. Generally, people prefer a healthy year of life immediately over one in the future. The WHO often uses a 3% time discount rate to years of life in the future, which implies that a year of healthy life gained 10 years from now is worth 24% less than a year gained now. For more details on the technical basis of the DALY, I refer to a paper by Murray (1994).
What does this mean for impact assessment?
It’s easy to incorporate DALYs in impact assessments: the methods are available in SimaPro and seem relatively straightforward to interpret. However, as you’ve read, different DALY calculation methods use different assumptions. ReCiPe and Impact2002+ do not take social preferences into account when calculating DALYs. In contrast, the Eco-indicator 99 method does account for these preferences, even if they are culturally influenced. As a result, the different methods give very different absolute results, although the relative differences between impact categories may not vary as much. In any case, it is important to keep in mind that DALYs from different sources may not be the same and can therefore not be summed.
So, how important is it to use DALYs in LCA? Aren't there a lot of subjective aspects to the calculation? The subjective aspects of the DALY are just a few out of many unavoidable choices you need to make in impact assessment. Other famous examples: how much scientific evidence do you want? What’s your time perspective – do you assume metals will have a toxic impact over the next 100 years or the next 10,000? These choices may have much more impact on your results than your choice of DALY methodology.
If you want to use DALYs to quantify human health damage, the best approach is to simply choose a single method and make sure to use it to compare the relative difference between options, products or life cycle stages, not as an absolute measure. Then, this metric can serve a crucial role in expressing human health damage, while not taking up valuable time and energy which you can spend on other, more important choices.
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