How old is the microplastic? A new way to estimate age

Fukuoka, Japan—Researchers from Kyushu University and the Asahi Kasei Corporation have developed a new way to estimate the age of microplastics found in the upper oceans. The method involves a combination of analysis of the plastic’s oxidation level with environmental factors such as UV exposure and ambient temperature.

Fukuoka, Japan—Researchers from Kyushu University and the Asahi Kasei Corporation have developed a new way to estimate the age of microplastics found in the upper oceans. The method involves a combination of analysis of the plastic’s oxidation level with environmental factors such as UV exposure and ambient temperature.

The team applied their new method to estimate the age of microplastics found at nearshore and offshore sites in the North Pacific Ocean. They found that the age of microplastics in nearshore areas ranged from 0 to 5 years, while those in offshore areas ranged from 1 to 3 years. Their findings were published in the journal, Marine Pollution Bulletin.

In marine environments from lakes to oceans, plastic is the most abundant type of pollutant. When plastic waste is exposed to the elements, they eventually break down and break down. Plastic waste that has decomposed into less than 5 mm is called ‘microplastic’.

“Microplastic pollution is recognized as a global problem. In previous studies, we found that there are about 24 trillion microplastic grains floating in the surface layers of the oceans,” explained Professor Atsuhiko Isobe of the University of Kyushu’s Research Institute of Applied Mechanics, who led the research. “However, we still know little about their effects on the environment or living things. The other big question we have is how long does it take for microplastics to stay in the oceans.”

To find out how old microplastics found in the oceans are, Isobe and his team started by investigating what metrics could be used to age microplastics.

“The most common material in plastics is called polyethylene. We know that polyethylene interacts with the environment, oxidizes and degrades,” explained Rie Okubo, a researcher at Asahi Kasei Corporation and first author of the study. “The degree of this degradation can be measured using changes in the molecular weight of the material and something called the carbonyl index. Simply put, when polyethylene is degraded, its carbonyl index increases and its molecular weight decreases.”

Of course, that’s not enough. Because microplastics are exposed to the elements, the team also needed to standardize how temperature and UV radiation affect plastic degradation. The team first conducted a series of exposure experiments on polyethylene materials and collected data on how various combinations of UV and temperature affect the material’s molecular weight and carbonyl index.

The team found that UVER—ultraviolet erythemal radiation, a measure of UV radiation at the ground surface—and seawater temperature were the two biggest contributors to plastic degradation.

“Once we had this data, we started applying it to our microplastic samples. All of our samples come from the upper ocean, up to one meter from the surface of the water,” continued Okubo. “We also collect microplastics from various regions. Several samples were collected close to the coast to Japan, ranging from 10 to 80 km offshore. Another sample was collected offshore, in the middle of the North Pacific Ocean and the Philippine Sea.”

By analyzing the collected microplastics, the team was able to estimate the age of each induvial sample. They found that nearshore microplastics were between 0 and 5 years old, while offshore samples were between 1 and 3 years old.

“We hypothesize that the reason why nearshore microplastics range from 0 to 5 years is because they often wash ashore and ‘survive’ for a longer time. On the other hand, offshore microplastics take longer to reach that part of the ocean, therefore we did not find any microplastics that are more than 3 years old,” explained Okubo. “These offshore microplastics are also likely to be lost from the upper ocean by depositing deeper into the water.”

The researchers hope this new method will give them better insight into how microplastics are generated and dispersed in the environment. The data will also help develop more accurate simulations for tracking microplastics across the ocean.

Isobe concluded, “Our research and understanding of microplastics is still very new, and thanks to this data we have gained some understanding of the basic science of microplastics. Our next step is to investigate how mechanical stimuli such as ocean waves and currents can degrade plastic, so that we can collect more accurate data.


For more information on this research, see “Estimated age of polyethylene microplastics collected from the oceans: Application in the western North Pacific Ocean,” Rie Okubo, Aguru Yamamoto, Akihiro Kurima, Terumi Sakabe, Youichiroh Ide, Atsuhiko Isobe Marine Pollution Bulletin

About Kyushu University
Kyushu University is one of Japan’s leading research-oriented institutes of higher education since its founding in 1911. Home to approximately 19,000 students and 8,000 faculty and staff, Kyushu U’s world-class research center covers a wide range of subject areas and research fields, ranging from the humanities and arts to engineering and medical science. Its campuses—including one of Japan’s largest—are located around Fukuoka City, a coastal metropolis on the southwestern Japanese island of Kyushu, often ranked as the world’s most liveable city and historically known as Japan’s gateway to Asia. Through Vision 2030, Kyushu U will ‘Driving Social Change with Integrative Knowledge.’ Its synergistic application of knowledge will encompass all academics and solve problems in society while innovating new systems for a better future.

About Asahi Kasei
Asahi Kasei Group contributes to life and lives for people around the world. Since its founding in 1922 with the ammonia and cellulose fiber business, Asahi Kasei has grown consistently through the proactive transformation of its business portfolio to meet the evolving needs of every era. With more than 46,000 employees worldwide, the company contributes to a sustainable society by providing solutions to global challenges through its three business sectors Materials, Homes and Health Care. For more information, visit

Asahi Kasei is also dedicated to sustainability initiatives and contributes to achieving a carbon neutral society by 2050.
To learn more, visit

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