Ionizing radiation was discovered by German physicist Wilhelm Conrad Rontgen in 1895. Since then, radiation has been used in various industries such as manufacturing, agriculture, health, and high technologies [1]. Accordingly, more and more industry sectors are using radiation after understanding its characteristics and energy properties. Radiation technologies are being more widely used in the area of research and development for daily necessities, including the health industry, advanced parts manufacturing, biotechnology, and new variety development [2]. With the use of radiation, the quality of life has been improved along with development of industries and economies [3]. On average, our radiation exposure due to all natural sources amounts to about 2.4 mSv a year [4]. It is considered that natural radiation has a minimal effect on the human body. However, the International Commission on Radiological Protection has warned that risks of radiation exposure are likely to increase proportionately with higher exposure to radiation even in small doses [5]. It has also been reported that plants growing in a soil rich in natural radionuclides coming from uranium or thorium can play a part in internal exposure to radioactive elements that we take into our bodies [6]. Phosphate rock, a pre-concentrated phosphate ore, is the primary raw material for the production of mineral phosphate fertilizer. It contains 0.02% of uranium. In its by-product, phospho-gypsum, radioactive elements including radium exist. Caesium-137 is one of the radionuclides from various human activities, such as the spread of nuclear materials, for some reasons. It can cause food contamination and internal exposure to radiation [7]. After the Fukushima Daiichi accident in 2011, massive efforts have been put together to regulate and decrease ¹³⁷Cs in food items in Japan [8–10]. There are pertinent risks of carcinogenesis related to internal radiation doses due to food contaminated by radioactive materials including ¹³⁴Cs, ¹³⁷Cs, and ¹³¹I according to Korea National Health and Nutrition Examination Survey. Nuclear-fuel reprocessing plants, nuclear wastes, and geological repositories are main sources of radioactive iodine in the environment [11, 12]. Conventionally, radiation exposures to uranium, radium, thorium, and potassium are well accounted for due to their roles in radioactivity from the environment. However, after recent accidents such as the Fukushima Daiichi accident in 2011, it is now of dire importance to measure radiation levels in Southeast Asia generally and Korea specifically due to its very close proximity to Fukushima, Japan [13, 14]. Iodine-131 is a very critical radioisotope which is closely associated with nuclear fission reactions. It causes environmental hazards in case of nuclear bomb attacks/mishaps and nuclear power plant accidents [15]. In case of a disaster or incident involving the spread of nuclear waste or a nuclear attack, the resultant radioactivity can spread up to 300 miles. The most extravagant health hazard from exposure to ¹³¹I is an enhanced risk of cancer induced due to radiation. It can also result in deformities, non-cancerous growths, and thyroiditis [16]. Accidents such as nuclear-reactor incidents at Chernobyl in 1986, the incident of Three Mile Island in 1979, and the most recent Fukushima nuclear reactor accident in 2011 provide clear evidence that it is urgent to understand health hazards posed by the presence of iodine in the atmosphere [17]. After the Fukushima incident, Korean authorities are taking threats of food-based radiation very seriously. The Ministry of Food and Drug Safety in Korea has conducted surveys to measure radioactivity levels of ¹³⁴Cs, ¹³⁷Cs, and ¹³¹I through the Korea National Health and Nutrition Examination Survey [12]. Geographically, Korea is the closest country to Japan. Therefore, there is a dire need to confirm the absence or presence of ¹³¹I in the environment and soil. Although the quantity of radiation by each radioactive element individually can be below the standard hazardous level or the maximum tolerance of 370 Bq/kg for any food, fertilizers with multiple radioactive elements used for farming might deliver radiation to agricultural products and consequently contaminate water reserves such as streams and groundwater after mixing with rain or irrigation water [6, 18–22]. Although there are a number of studies available about radioactivity due to ²³⁸U, ²³²Th, and ⁴⁰K present in chemical fertilizers from countries such as Serbia, Bangladesh, India, and Iraq [23–26], studies that measure the radioactivity of agricultural fertilizers in Korea and the rest of the world are lacking. Studies generally involve ¹³⁷Cs and ¹³⁰I specifically. A variety of reports have analyzed radioactivity levels in water, air, and soil [27–29]. Hence, this study will serve as a foundation to better monitor the impact of radioactive levels in chemical fertilizers on public health and environment and to ensure that radiation levels in chemical fertilizers that contain essential nutrients for plant growth and development are safe for human health and the environment.

The nitrogen fertilizer had 3.49±5.71 Bq/kg of ¹³⁷Cs, which was the second highest concentration, 34.43±7.61 Bq/kg of ¹³⁴Cs, and 569.16±91.15 Bq/kg of ⁴⁰K, whereas ¹³¹I was not detected. The calcium fertilizer had the highest concentration of ¹³⁷Cs at 5.74±4.40 Bq/kg. It also had 22.37±5.39 Bq/kg of ¹³⁴Cs and 433.67±64.24 Bq/kg of ⁴⁰K. However, ¹³¹I was not detected. The sulfur fertilizer had 347.31±55.73 Bq/kg of ⁴⁰K, 19.42±4.53 Bq/kg of ¹³⁴Cs, 2.21±3.49 Bq/kg of ¹³⁷Cs, and 0.04±0.22 Bq/kg for ¹³¹I. The phosphoric acid fertilizer had the highest concentration of ⁴⁰K at 70,007.34±844.18 Bq/kg and 46.07±70.40 Bq/kg of ¹³⁴Cs, whereas neither ¹³⁷Cs nor ¹³¹I was detected. The potassium chloride fertilizer had 12,827.92± 1,542.19 Bq/kg of ⁴⁰K and 94.76±128.79 Bq/kg of ¹³⁴Cs. Neither ¹³⁷Cs nor ¹³¹I was detected (Table 4). Fig. 3 shows radioactive concentrations and spectra of the five fertilizers and distilled water measured for 3,600 seconds.

Currently, radioactive substances manufactured or processed using radiation technologies are being accumulated worldwide. They are mostly radioactive wastes produced from nuclear power plants, fallouts from nuclear tests, and radioisotopes used in the health industry [30]. Due to some nuclear disasters such as the Chernobyl nuclear disaster in 1986 and the Russia nuclear disaster in 1993, people are concerned about impacts of radiation on human health [31]. As many people are exposed to radionuclides from natural radiation sources in the soil due to such accidents, it is important to minimize radiation impacts. Zmazek et al. [32] have reported that areas rich in granite are more likely to have uranium and radon than non-granite areas. Granites include 2–12 parts per million (ppm) of uranium and 8–33 ppm thorium [33]. Every year, the Korea Institute of Nuclear Safety measures radiation in subsoil and topsoil for 14 areas and publishes analysis results due to concern about radiation impacts. Thus, the objective of this study was to measure radiation levels of inorganic fertilizers widely used in agriculture. As a result, ⁴⁰K showed the highest levels in fertilizers containing phosphate acid and potassium chloride. A phosphate acid fertilizer’s base material is phosphate rock, a natural radiation source which contains ²³⁵U, ²³⁸U, and ²³²Th. Phosphate rock exporters to Korea are Morocco, China, Israel, and Togo. It was found that rocks they exported exceeded the radiation standard (1,000 Bq/kg) [34] in terms of ²³⁵U and ²³⁸U. As potassium chloride is added in order to improve hygroscopic capacity in the process of manufacturing a phosphate acid fertilizer, the level of ⁴⁰K increases accordingly [35]. A potassium chloride fertilizer is manufactured with leucite, alunite, and sericite known to be rich in potassium. It has been reported that ores rich in potassium imported by Korea from countries such as Canada, Belarus, and Russia have high ⁴⁰K levels, exceeding 1,000 Bq/kg. Almost all foods have natural radioactivity due to radioactive isotopes. For example, ⁴⁰K is present at 40–50 Bq/kg in cows’ milk, 400–500 Bq/kg in milk powder, 600–800 Bq/kg in concentrated fruit juice, and over 1000 Bq/kg in instant coffee [22]. Natural radioactivity is primarily due to primordial radionuclides ⁴⁰K, ²³²Th, ²³⁸U, ²³⁵U, and so on present in different quantities depending on soils of different regions in the world [36]. According to the International Atomic Energy Agency, a permissible limit for food is 370 Bq/kg of ⁴⁰K [35]. However, Rajacic et al. [25] have demonstrated that the concentration of ⁴⁰K in fertilizer is 4,860 Bq/kg [25]. Radiation due to ⁴⁰K, a radionuclide, is rapidly absorbed into the body, causing internal exposure to radioactive elements that we take into our bodies through food and water [37]. In the environment, ⁴⁰K is redistributed through agriculture by application of fertilizers. Thus, its concentration should be measured to distinguish safe utilization levels of fertilizers rich in ⁴⁰K [19, 25, 38]. From production to utilization in agriculture, fertilizers might cause exposure to gamma radiation (external exposure) [39–41]. Many biomedical hazards are associated with exposure to radiation for a few weeks or months to years, which might cause radiation-induced liver disease [42–45]. Exposure to a significant level of radiation released from fertilizers can potentially cause cancers. Thus, a deeper understanding of fertilizers-based radiation is required [46, 47]. In particular, ⁴⁰K is a constant and uniform source of exposure to radiation for humans [48]. The present study examined inorganic fertilizers produced by a single manufacturer. Results might be different depending on the country and areas from which fertilizers are imported. Further studies about inorganic fertilizers in more details are needed to create measures to reduce ⁴⁰K.

As a result of this study, ⁴⁰K had the highest levels in five fertilizers. Thus, there is a need to take measures to reduce radiation exposure from ⁴⁰K contained in fertilizers including phosphoric acid and potassium chloride. This study can help us understand radioactive concentrations in five inorganic fertilizers used in Korea. In the future, it is necessary to set up measures to cut radiation impacts from radionuclides.