Hannah Baker
Camborne Science & International Academy
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Introduction and context
In the UK, during 2023, around 7.7 million single-use vapes were purchased each week. With 11% of adults in the UK vaping regularly, it poses the question: how were the 400 million vapes purchased in 2023 disposed of?
One study found that 83% of vape users were not disposing of them in a correct and sustainable manner, with 7% of vape users admitting to dropping the vapes on the street. Currently, the guidance is to return used vapes to shops from which they were purchased, where they are able to recycle the devices. However, this system is greatly underused, with the high rate of littering in the UK it is not uncommon to see discarded vapes on the street or in natural parks and green environments.
As a result, we questioned how any remaining e-liquid within the vapes would affect invertebrates, should it leak into an aquatic environment. E-liquid contains a variety of chemicals including: vegetable glycerin, propylene glycol, nicotine and flavourings including monosodium glutamate. Although vegetable glycerin does not directly affect D. magna it can affect the water quality in which they live, decreasing the invertebrates’ chance of survival. Propylene glycol is acutely toxic for D. magna and can have a significant epigenetic effect on their reproductive cycles. Nicotine and monosodium glutamate are discussed later in this article. Nicotine is restricted to a maximum concentration of 20mg/mL in the UK by law.
We used Daphnia magna as the model invertebrate. This is because they are transparent which enabled us to see their heart beating. They are also easy to keep and breed in a lab. Lastly, there are minimal ethical requirements around testing D. magna. Despite this, to reduce the amount of harm caused to them, we did the fewest number of tests possible to get our results and minimised tests at high concentrations of e-liquid. We also only kept them in the lab for 2 weeks at a time.
Method
During the experiment, we aimed to measure both the heart rate and relative movement of D. magna in different e-liquid concentrations. Each time we did an experiment, we would measure the heart rate and movement of the D. magna in 0% e-liquid, which we could compare the results involving e-liquid too. This was to try and regulate extraneous variables which we could not control, such as the age of the D. magna.
Firstly, we diluted the e-liquid with distilled water to create the desired concentration for the experiment. We then placed a minimum of three D. magna in the e-liquid solution and left them for 5 minutes. After 5 minutes, we observed the movement. It was given a semiquantitative value between 0 and 5, where 5 was the level of movement of D. magna in the control solution. Then to measure the heart rate we counted it under a microscope.
During the experiment the following extraneous variables were controlled: temperature of solution, time exposed to e-liquid, method of counting the heart rate, and the nutrients available to the D. magna.
At each different concentration of e-liquid that we investigated, tests were carried out on a minimum of 3 D. magna and a mean was calculated for the heart rate after removing any anomalous results.
Results
After collecting all the data on D. magna’s heart rate at different percentages of e-liquid, the graph below was plotted (Figure 3a). It illustrates the mean D. magna heart rate measured at the different percentage of e-liquid we tested. The exponential line of best fit depicts a decreasing trend in the D. magnas’ heart rate as the percentage of e-liquid increases, until it eventually reaches zero. There was an value of 0.92. This suggests that there is a 92% probability that increasing percentages of e-liquid in the water would have the effect of decreasing the D. magna’s heart rate and thus affecting them.
Figure 3b portrays the D. magna’s relative movement at different percentages of e-liquid. A peak can be observed on both graphs at 3.5% e-liquid. Initially this was thought to be caused by an error in the data, however, as we continued to test the D. magna’s heart rate and movement at this percentage, it was consistently higher than expected. It is thought that this peak is therefore due to one of the ingredients within e-liquid. It can be seen that many of the ingredients have the effect of decreasing the heart rate of D. magna, most prevalent being nicotine. However, monosodium glutamate has been shown to increase the heart rate of D. magna, so this could be causing the peak in heart rate, before other ingredients overtake and cause the heart rate to begin decreasing again.
Conclusion
Increasing concentration of e-liquid within the aquatic environment will have the effect of decreasing the heart rate of invertebrates, such as, D. magna. However, certain ingredients within e-liquid, for instance, monosodium glutamate, could have the effect of increasing their heart rate instead.
There needs to be clear instruction about how to dispose of e-liquid and vapes in order to reduce the amount of e-liquid chemicals that end up in ecosystems. E-liquid could also be adapted to contain chemicals which have a smaller effect on invertebrates, should it leak into our natural environment.
Further Research
To create a more environmentally sustainable e-liquid product, it would be necessary to understand how individual ingredients are affecting the invertebrates. Therefore, further research is required in understanding what effect each chemical has.
Biomagnification is a problem within ecosystems where toxic chemicals, which are absorbed by invertebrates or smaller organisms, are passed up the food chain. As larger consumers eat a large number of smaller organisms, the amount of e-liquid could build up and be much more significant in the body of the consumer. It would be interesting to investigate to what extent this would take place.
Lastly it would be interesting to investigate how longer periods of exposure to very small concentrations of e-liquid could affect invertebrates. This is because the e-liquid within aquatic environments is likely to be in extremely small concentrations. To test what these concentrations would be it would be necessary to test natural water sources in the UK, to see if any of the chemicals could be detected.
References
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California State Science Fair. (2017). Project summary J2220. Retrieved from https://csef.usc.edu/History/2017/Projects/J2220.pdf
Dennis, P. (2023). 78% of single-use vapes disposed of in general waste bins. Circular Online. Retrieved from https://www.circularonline.co.uk/news/78-of-single-use-vapes-disposed-of-in-general-waste-bins/
Jackson, S. E., Beard, E., Shahab, L., Brown, J., & West, R. (2025). Nicotine strength of e-liquids used by adult vapers in Great Britain: A population survey 2016 to 2024. Addiction, 120(3), 468–482. https://doi.org/10.1111/add.16576
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Perez, K., Lucas, C., Jeffries, B., & Legg, T. (n.d.). Increasing heart rates of Daphnia magna in an excitatory monosodium glutamate solution versus decreasing heart rates in a depressive ethanol solution. Retrieved from https://undergradsciencejournals.okstate.edu/index.php/JUBLI/article/download/9693/2023
Thomas, T. (2024, August 12). Number of UK adults who vape reaches record level, report finds. The Guardian. Retrieved from https://www.theguardian.com/politics/article/2024/aug/12/number-of-uk-adults-who-vape-reaches-record-level-report-finds
