5.9.6 Ionizing radiation

Ionizing radiation exposures can cause cancer of different types depending on the type of radiation and the tissue receiving the highest dose (IAEA, 1996b). Following a nuclear accident and resultant increased exposure to radiation, cancer rates in the exposed population may rise, as was the case after the atomic bomb explosions in Hiroshima and Nagasaki (HICARE, 1993). The latency period will vary depending on the type of cancer (Fig. 5.12).

The most serious nuclear accident to date occurred at Chernobyl on 26 April 1986, releasing large amounts of radioactive material over vast areas of Belarus, Ukraine and the Russian Federation (WHO, 1996j). One of the main constituents of this release was a radioactive isotope of iodine- 131. Four years after the Chernobyl accident, increased incidence of thyroid cancer was observed in children in the most highly exposed areas. This was much sooner than would have been predicted from the studies of Japan's atomic bomb survivors (Fig. 5.12). Thyroid cancer is normally rare in children, but more than 700 cases of thyroid cancer in children under 15 years of age were reported between 1986 and 1996 (Table 5.11). Thyroid cancer incidence among children in an area of Belarus, with the highest radiation deposition, was about 200 times greater than that observed in England and Wales during the same time period (see also Sections 4.5.2, 5.7.3 and 5.7.4).

Fig. 5.12 Latency period for development of malignant tumours

Source: HICARE, 1993.

Table 5.11
Thyroid cancers in children (under 15 years of age) in Belarus, Ukraine and the Russian Federation after the Chernobyl accident

Year	Belarus	Ukraine	Russian Federation (Bryansk and Kaluga regions)
1986	2	8	0
1987	4	7	1
1988	5	8	0
1989	7	11	0
1990	29	26	4
1991	59	22	3
1992	66	47	5
1993	79	43	11
1994	82	39	19
1995	91	44	9
Total	424	255	52

Sources: Nagataki & Yamashita, 1996; Tsyb et al, 1996; WHO, 1996k; Williams et al., 1996; Troako, 1997.

An increase in leukaemia incidence among children has not yet been observed in the contaminated areas around Chernobyl, or in any other European country modestly exposed to radiation fallout from the Chernobyl accident. Nevertheless, if patterns observed following the dropping of atomic bombs on Hiroshima and Nagasaki offer any indication (Fig. 5.12), a higher incidence of leukaemia, colon cancer, breast cancer and lung cancer may eventually occur.

In recent years, possible cancer risks associated with exposure to radon in houses have received attention. It is estimated that in the USA about 20000 cases per year of lung cancer occur as a result of radon exposure (US Department of Health and Human Services, 1991). Smoking considerably increases the risk of lung cancer related to radon, and vice-versa (WHO, 1996j) (see also Section 4.2.6).