Nanoparticles and microplastics - real threats or toxicological fashions

Lauantai 13.1.2024 klo 19.21 - Mikko Nikinmaa

In the beginning of 2000s nanotoxicology became very popular. Before 2004 there were no Web of Science articles, in the 5-year period 2004-2009 175, 2010-2014 slightly above 1000 and between 2015 and 2019 about 2100. The results clearly showed that nanomaterials can be toxic. However, virtually all studies were done with nanomaterial levels which far exceeded environmentally relevant levels. Once it was established that nanoparticles can be toxic, it has become clear that their effects in nature must be demonstrated before they can be considered to be an ecotoxicological problem. Luckily it appears not to be the case as the number of nanotoxicological studies has decreased markedly, to about 900, in the last five years.

One can naturally hope that all the studies which demonstrated the possibility of toxic effects were enough to alert the people responsible for the disposal of nanomaterials about the need to consider how the waste is treated. If that were the case, toxicological research had reached a major goal: preventing a potentially significant environmental problem from developing. In worse case, the situation shows that a lot of scientists eagerly follow the fashion and study something that is popular without considering its importance. What the results have shown is that nanoparticles are probably entering the cells of organisms via, e.g., pinocytotic pathways. Thereafter the effects are probably due to the toxicity of the compound(s) which the particle is made of, and environmental impact depends on the probability of nanoparticle concentration reaching a level which causes malfunction of some organisms.

Thus, nanoparticle toxicity is really the toxicity of the material that the particle is made of, the particle itself is only a means of entering the cells. For example, charged compounds are virtually impermeant, but if they are nanoparticle components, they can be taken up via pinocytosis. If the charged compound thus entering the cell is toxic, it will be harmful. So instead of nanotoxicology addressing all nanomaterials, we should evaluate, which toxic materials are used in nanoparticle formulation. If a material is inert, toxic effects are not likely.

About five years after nanotoxicology became in fashion, the same happened to microplastics. Between 2011 and 2015 there were merely 290 studies about them, but during the past five years close to 14000. Most of the studies are concerned with the distribution and uptake of the microplastics. They have now been found everywhere, from Antarctica to glaciers in the Alps and within virtually all organisms, mothers’ milk etc. Their occurrence everywhere is quite clear, but actually very little is known about their toxicity. In contrast, plastic waste, macroplastics has various harmful effects in addition to fouling the environment. Animals get stuck to plastic waste, plastics can clog their lungs, gills or intestine. To everyone watching news reels the sorry sights of seals and birds having died in nets is familiar. But microplastics, their effects are uncertain.  

If microplastics are made of pure polyethene or polyethylene, I have difficult to see that they could be toxic. They are inert materials which can, in my opinion, best be compared to cellulose or starch, which we and all the herbivores eat and digest all the time. However, as with nanoparticles, microplastics can be toxic, if they are associated with toxic materials. This is possible or even probable, if the environment is polluted with hydrophobic toxicants. They get adsorbed to plastics instead of any aqueous material. The toxic particles can then be taken up via pinocytosis and exert cellular toxicity. However, again as with nanoparticles, microplastics in themselves are not toxics, they are just the means by which hydrophobic toxicants can get in the tissues.

As a conclusion, I must stay that many of the studies on nanoparticles and microplastics have been done because the topics are fashionable. Instead of showing over and over again that nanoparticles can be toxic or that microplastics are found everywhere one should start considering, when their presence causes environmental risk because of their association with the pollutants which are the true problem.

Fashion largely determines what is studied in (aquatic) toxicology

Perjantai 18.10.2019 klo 17.37 - Mikko Nikinmaa

I finished as editor-in-chief of Aquatic Toxicology at the end of July after 14 years. During that time, I have handled more than 6000 manuscripts, which makes it possible to evaluate, what the scientists are studying. It also gives an indication about what is funded, since adequate funding is a prerequisite of being able to carry out the research. Overall, I must say that I am disappointed, since it appears that the funders mainly support fashionable topics, and scientists are naturally willing to do what gives them funding. Associated with this is the positive correlation between fashionable topic and the impact factors of journals. If you have many articles in a journal on a fashionable topic, its impact factor increases, even if the real environmental relevance of the work were poor. I present some major problems, which are the result of trying to do fashionable things instead of thinking already at the outset, what the real environmental relevance of the studies is.

First, it seems that using the newest possible methodology enables you to do work, which has little importance, and still get funded. In the past 15 years the -omics methods have increasingly been utilized by environmental toxicologists. Although they give new possibilities, if properly interpreted and utilized, their improper use is common, and many conclusions are faulty. Most studies use a very small number of organisms, typically 3. This is far too small number for any conclusions with natural populations of animals, especially as their environmental responses may involve changes in variability. I suppose everybody accepts that the responses to toxicants depend on the functions of proteins and their disturbances. Yet, most studies forget this, and based on real-time PCR, microarray or RNA-seq data, which show an increase in steady-state mRNA level conclude that the function encoded by the gene studied has increased. However, this need not be so: if the protein activity decreases because of the action of a toxicant, transcription is increased as a compensatory response. Yet, even after the compensatory response, protein activity may be reduced. In fact, some studies have seen this happening, but have not indicated this obvious explanation just being surprised of the finding. While the above concerns the commonly used transcriptomics, one can find problems with proteomics and metabolomics also. Basically, since toxicants can only affect organisms, if they disturb some functions, functional measurements are required. The -omics data help in finding the genes and consecutively functions, which may be affected. The reason why this is seldom done is twofold: functional measurements are time-consuming, and it is hard to make them high-throughput; the methodology is usually classical and does not attract funders as the use of fancy methodology does.

Second, nanotoxicology was in fashion a couple of years back. Between 2010 and 2015 one could publish virtually anything showing that nanomaterials can be toxic. In most cases, the amount of nanomaterial used has little bearing to what the environmental levels are or may be in future. Yet, relating the toxic actions to nanoparticles to their environmental occurrence is virtually undone. I fear that the same is happening with the new fashionable topic: microplastics. Horror stories are told about the effects of microplastics. Yet, virtually nothing is known about the effects of environmentally occurring levels of microplastics on the function of organisms.

Third, climate change and interactions of toxicants with temperature or oxygen level, or other environmental variables has hitherto been understudied. This knowledge gap is presently being filled. However, a significant problem remains, and most studies do not even indicate its existence. The studies are typically short, often 1-20 days, and the temperature, carbon dioxide or oxygen level change are typically imposed with virtually no lag time using values expected to occur a hundred years from now. This means that the stress levels in the studies are completely different from naturally occurring ones.

Finally, we are suffering from the tyranny of the mean. Virtually always a toxicological response is considered to be a change in the mean of a parameter. Changes in variance are virtually never considered as a toxicological endpoint. One is considering the heterogeneity of data only as determining if data transformation is needed for statistical testing. Yet, when I went over many toxicological studies, I observed that in most of them variability changed without a change in mean. In those cases, variability is undoubtedly a more sensitive indicator of a toxicological response than the mean. We have pointed out the possible importance of variability as a toxicological end point (Nikinmaa, M., Anttila, K. Individual variation in aquatic toxicology: not only unwanted noise. Aquatic Toxicology 207, 29-33; open access)

Toxicity of Nanoparticles - Hype or Reality

Sunnuntai 8.4.2018 klo 12.27 - Mikko Nikinmaa

During the recent past, the toxicity of nanoparticles (i.e. particles with at least one dimension less than 100 nm) has become a very fashionable field of toxicological studies. There is now ample evidence that the particles can be toxic, if their concentration is high enough. And that is the major problem of most nanotoxicological studies: the nanoparticle levels are often thousands of times higher than what can be expected to occur in the environment. Since one has now clearly shown that nanomaterial can be toxic, it would be high time to study the possible environmental relevance of the toxicity. If there is none, then the studies showing toxicity are irrelevant. This is because one can find toxic amount of any substance. For example, one can demonstrate a lethal dose for water. As Paracelsus said already in 16th century: All substances can be poisonous, the dose makes the difference between remedy and poison.

A significant problem with nanomaterial studies is that the methodology used is suitable especially for dissolved substances in aquatic media, but is not necessarily suitable for the new material. Hitherto, methods, which would be specific and good for nanomaterial research have not been developed. A significant property of nanomaterials is their tendency to aggregate, and the influence of this on the toxic properties is poorly described - it makes definitely a big difference if aggregation occurs before the contact with organisms or only after cellular uptake. One toxic effect of nanomaterials, which is independent of their metal components, is that they cause oxidative stress (and inflammation). This property may get worse with aggregation - we do not know. As the worst possible scenery one can think that nanomaterials cause similar problems in airways as asbestos: this may be fearmongering, but until environmentally relevant nanotoxicology studies are available, the possibility cannot be discounted.