Scientific Methodology

In general, degradation refers to the reduction of mass through mineralization, measurable by the production of CO₂ (or CH₄under anoxic conditions), or the consumption of O₂. However, in many environmental degradation studies, direct measurements of gas production are not possible, so mass loss is used as a proxy for degradation. Note that this may lead to overestimating degradation rates because it can include fragmentation or other losses. The degradation rates of different polymers used in our calculator are based on a thorough literature review.

Due to lack of data and models we use a purely surface driven model, i.e. an item is shrinking from the surface with a constant velocity, being aware that this is a simplification, because degradation might also occur in the bulk (e.g. by hydrolysis).

Although the absolute degradation velocity in our surface-driven degradation model is constant for each polymer type in each compartment, only films show a linear reduction of mass. On the other hand, for fibers and particles, degradation is faster at the beginning and slower towards the end, due to changing surface to volume ratio. This leads to a higher persistence for films compared to particles when starting from the same initial size.

The figure shows the surface degradation rates estimated so far.

Following the common structure of impact categories, the calculated impacts are expressed as a dimensionless factor (Characterization factor) compared to a reference material (e.g. Methan is 29-times “stronger” than Carbon dioxide, the reference material for global warming – 1 kg of Methan is equivalent to 29 kg CO2e emission).

However, with plastic pollution the definition of reference material is not as straightforward, because no dominant polymer exists, the polymers might behave very differently in different compartments and the degradation of polymers, especially very persistent ones, is very difficult to measure. Any uncertainty related to the chosen reference material would then spread to all characterization factors.

Therefore, we decided to use an artificial emission as reference material which has a residence time of 1 year. Division by this factor does not alter the numerical value but makes the number dimensionless [time / time] as it should.

The plastic pollution equivalent of an emission is than calculated by the real mass of the emission multiplied with the characterization factor.

The plastic pollution equivalent is given as a mass. However, this does not equal the real mass in the environment (Similar to global warming and CO2e). Due to the intentionally chosen fast degradation time of the reference material (1 year), the PPe is much higher than the mass of the real emission expressing the urgent need to reduce plastic emissions.

Often emissions are not a single event (e.g. 1kg), but a continuous flow (e.g. 1 kg/y), like the emissions of wastewater treatment plant, tire and road wear particles on a section of a street etc. In case of a steady state the hold-up in the environment which is necessary to balance the continuous flow. can be easily calculated by the characterization factor PPe due to the chosen reference time of 1 year.

The hold-up equals the yearly flow times the characterization factor. E.g. an emission of 100 kg/y and a residence time of 75y (or characterization factor of 75) generates a hold-up of 7500 kg to balance the emission in the steady state. Please note, than this hold-up is the real mass in the environment and not a fictional one as described above for the PPe!

A world without plastics is not feasiable. Although, it is clear that emissions must be reduced as far and as soon as possible, a certain amount of plastic emissions might to be tolerated. The PPe can be used to calculate the "allowed" yearly emission worldwide or per captiva. (Plastic Pollution Budget)

Assuming that 9.5 billion tons of plastics (thermoplastics, thermosets, elastomers and man-made fibers) have been produced on a global scale so far and that the loss can be estimated at around 3%, a total of 285 million tons have already entered the environment. Assuming that some degradation has already occurred (especially of rubber), an estimate of 250 million tons of plastic currently in the environment seems reasonable and rather conservative.

If this corresponds to the maximum amount of plastic that can be tolerated in the environment, the PPe definition suggests that an emission of 250 million tons of PPE per year would be permissible for the entire world population (7.9 billion inhabitants). This corresponds to the global plastic pollution budget (global PPB).

Assuming the same individual budget for each inhabitant, this results in a value of 31.9 kilograms of PPe per person per year (31.6 kgPPe/(cap a)) (individual PPB). In the case of poorly degradable plastic emissions (CF = 100), this results in an allowed emission of 316 grams in real mass per year; in the case of quickly degradable plastic emissions (CF = 1), of 31.6 kg per year. In the long term, both emissions lead to the same final quantity in the environment, although it takes different amounts of time to reach this steady state.

Please note, this is only a model calculation to determine a global amount of plastic emissions. Due to the exponential growth of plastic production and the long persistence (centuries) of most polymers, the actual situation is by far not stationary! Even if the PPB would be fulfilled, the amount of plastic pollution would first overshoot dramatically.

Data regarding plastic production in 1950 to 2015 are based on: Geyer, Roland; Jambeck, Jenna R.; Law, Kara Lavender (2017): Production, use, and fate of all plastics ever made. In: Science advances, 3(7), e1700782.

Life Cycle Assessment (LCA) is a methodology used to estimate potential environmental impacts of products and processes. The impacts form so called impact categories - global warming caused by greenhouse gas emissions as the most popular one.

For plastic products LCA studies often show lower environmental impacts compared to alternatives, e.g., due to light-weight design or a lower resource input (Amienyo et al. 2013; Humbert et al. 2009; Saleh 2016) while plastic emissions caused by loss of plastics, e.g., by abrasion, aging, fragmentation, or littering (Sonnemann and Valdivia 2017), are currently not well considered by common LCA methodologies.

While there is a clear consensus between politicians, industry, and consumers that plastic should not be released to the environment (Nielsen et al. 2020), they can be found in nearly all ecosystems worldwide (Li et al. 2020), possibly threatening biodiversity and ecosystem services, such as fish reproduction, which negatively influence the economy, such as the fishery or tourism industry (Burns and Boxall 2018). Future LCA, therefore, needs to consider emissions of plastics into the environment and its potential environmental impacts (Schwarz et al. 2019; Sonnemann and Valdivia 2017; Woods et al. 2016).

Introducing a new impact category for plastic pollution and using the plastic pollution equivalent as a reference unit it is possible to consider different plastic emissions to the environment in LCA calculation at least on a midpoint level.