# The Impact of Tritium in the Environment
## 4 Biological Effects  
Being analogous to protium, except for in certain reactions where the difference in behavior is due to the mass of \({}^{3}\)H, tritium is included in the hydrogen cycle in the biosphere, in the behavior of HTO and its vapor form, and in various organic and hydrogen compounds. The ecological interest in tritium is due to three main factors:  
1. The rather high content of tritium in the biosphere due to the combined influence of natural and anthropogenic factors. 2. The extremely high rate of \({}^{3}\)H's incorporation into biochemical processes. 3. The potential radiation hazard to living organisms, primarily from the standpoint of genetic effects [1]. The beta energy of tritium is significantly lower than the gamma-ray energy of major gamma-emitting radionuclides. Tritium does not pose a significant hazard in the case of external human exposure, since the maximum energy of the beta particle formed during its decay is insufficient to penetrate the skin. This beta particle's calculated and measured maximum travelling distance in water does not exceed 6 \(\upmu\)m (Table 2). At the same time, the outer stratum corneum of the skin is 10 to 100 \(\upmu\)m thick, the epidermis is 100-200 \(\upmu\)m thick, and the next layer is 1000-3000 \(\upmu\)m thick [52]. Therefore, the main ways of \({}^{3}\)H entering the human body include the consumption of drinking water, food containing the aqueous form of or organically bound tritium (OBT), inhalation, and sorption through the skin [53]. The contributions from the inhalation and the dermal absorption of HTO are equal. An analysis of the literature sources shows that the radiobiological properties of tritium have not been studied sufficiently. Uncertainty of different scientifically and experimentally grounded assessments considerably exceeds a factor of 100,000. Estimates of safe concentrations of tritium in drinking water, according to official WHO [56], EURATOM [57], ICRP [58], IAEA, and EPA [59] documents and scientific publications [46], differ by a factor of almost 600,000 [60, 61, 62, 63, 64]. According to the US EPA, cancer risk coefficients (CRCs) for tritium are 9.44 \(\times\) 10\({}^{-13}\) Bq\({}^{-1}\) (mortality CRC) and 1.37 \(\times\) 10\({}^{-12}\) Bq\({}^{-1}\) (morbidity CRC) [65]. The recommended health criterion established in the USA for water and food consumption is a 10\({}^{-6}\) de minimis cancer risk level (CRL) [66]. According to [67], the recommended EC of a 1 \(\times\) 10\({}^{-6}\) lifetime risk of cancer for use in risk assessments means that in a population of one million people, one additional case of cancer will occur during their lifetime. The calculation of the safe concentration (C\({}_{1}\)) of tritium for health is as follows:  
\[\mathrm{C}_{1}=\frac{\mathrm{CRL}}{\mathrm{EP}\times\mathrm{CRC}\times\mathrm{ WIR}} \tag{7}\]  
where EP is the exposure period (70 years during a lifetime), and WIR is the water ingestion rate equal to 2 dm\({}^{3}\cdot\)day\({}^{-1}\) or 730 dm\({}^{3}\) annually for adults. Calculations show that the \"safe\" concentration of tritium in drinking water (C\({}_{1}\)) should be between 14 and 21 Bq\(\cdot\)dm\({}^{-3}\) to avoid exceeding the minimum risk of cancer morbidity and mortality, respectively. Numerous experiments on experimental animals (mice, rats) have made it possible to establish a No Observed Adverse Effect Level (NOAEL) of tritium in the range of (0.63-3.7) \(\times\) 10\({}^{7}\) Bq\(\cdot\)kg\({}^{-1}\) of body weight (BW) [68, 69, 70, 71]. The calculation of the \"safe\" concentration (C\({}_{2}\)) of tritium using the NOAEL is as follows:  
\[\mathrm{C}_{2}=\frac{\mathrm{NOAEL}\times\mathrm{BW}\times\mathrm{RSC}}{ \mathrm{UF}\times\mathrm{IR}} \tag{8}\]  
where the NOAEL is 3.7 \(\times\) 10\({}^{7}\) Bq\(\cdot\)kg\({}^{-1}\), which is the highest experimental value, as previously indicated, and BW is 70 kg for adults and 10 kg for infants (i.e., children under the age of 1 year).