# Presentation: 863234

## What Is Ionizing Radiation ?

## Electromagnetic Spectrum

## Alpha Particle (α)

**	****	****Alpha decay, **the emission of an alpha particle ( a helium nucleus), only occurs in very heavy elements such as uranium, thorium and radium. The reason alpha decay occurs is because the nucleus of the atom is unstable. In an attempt to reduce the instability, an alpha particle is emitted. The alpha particles are in constant collision with an energy barrier in the nucleus and because of their energy and mass, there exists a nonzero probability of transmission. That is, an alpha particle (helium nucleus) will tunnel out of the nucleus of the heavy element.

## Beta Particle (β)

**Beta decay** occurs when the neutron to proton ratio is too great in the nucleus and causes instability. In basic beta decay, a neutron is turned into a proton and an electron. The electron is then emitted.
- There is also **positron emission **when the neutron to proton ratio is too small. A proton turns into a neutron and a positron and the positron is emitted. A positron  is basically a positively charged electron.

## Gamma Ray (γ )

- 	After a decay reaction, the nucleus is often in an “excited” state. This means that the decay has  produced a nucleus which still has excess energy to get rid of. Rather than emitting another beta or alpha particle, this energy is lost by emitting a pulse of electromagnetic radiation called a gamma ray. The gamma ray is identical in nature to light or microwaves, but of very high energy. Like all forms of electromagnetic radiation, the gamma ray has no mass and no charge. 	Gamma rays interact with material by colliding with the electrons in the shells of atoms. They lose their energy slowly in material, being able to travel significant distances before stopping. Depending on their initial energy, gamma rays can travel from 1 to hundreds of meters in air and can easily go right through people. 	It is important to note that most alpha and beta emitters also emit gamma rays as part of their decay process. There is no such  thing as a “pure” gamma emitter.

## Radioactive Atom

_**Radioactive Atom**_

## Relative Stopping Power

## Effects

## DNA damage

## Sequence

- Sequence of events by which ionizing
-     radiation affects living systems
- Energy absorption (10**-17 **sec)
- Ionization and excitation (10**-5 **sec)
- Molecular changes (secs)
- Biochemical changes (secs - hrs)
- Physiologic & anatomic changes (min - hrs)
- Biological effects (hrs - yrs)
- Death of organism

## EPA Radionuclide Risk Coefficients

**Notes:**
The 1974 SDWA called for EPA to regulate drinking water contaminants in two steps:  1) to create National Interim Primary 

## Radioactivity vs. Dose

- Activity is disintegrations per second (dps)
- 	- Unit is picocurie (pCi) = 0.037 dps
- 	- Unique decay rates and half lives for each radionuclide
- 	- Depending on radionuclide, decay can be α or β,                  	usually with one or more γ
- 	- Energy per decay is sum of particle and gamma 	energies
- Dose is energy deposited in a unit mass of a medium (e.g., per gram of tissue)
- Same activity of different radionuclides ≠ same dose (depends on energy per decay)

## Converting Radiation Dose to Risk

- Dose assessments use effective dose (all tissues) for adult receptor
- Radionuclide slope factors are age-averaged -- calculate doses to each target organ or tissue, apply age- and gender-specific risk factors, and integrate over a lifetime
- External (uniform whole body) irradiation =
- 	8 x 10-7 radiogenic cancers (incidence)/millirem
- For intakes, dose to risk conversion is radionuclide-dependent and varies between
- 	~ 1 x 10-7 and 3 x 10-6 per millirem

## Radiation Risk Assessments -

- Do not use body weight
- Do not use averaging time
- Are “best estimates” of age-averaged risk to the U.S. population, not upper 95th %
- Are derived from human epidemiology (dose response data extrapolated from atomic bomb survivors, U miners, etc.)
- Are based on linear, no-threshold model (LNT)

## Health Basis for Radium MCL

**1976 MCL**Ra-226 and Ra-228 = 5 pCi/L combined
- Critical organ: Bone and soft tissues
- Based on NAS-BEIR 1
- Est. lifetime fatal cancer risk: 0.5 to 2.1 x 10-4
- Ra-226 and Ra-228 assumed to contribute equally to risk
- Consistent with 1962 FRC recommendation, that the daily intake not exceed 20 pCi.  Allowing 5 pCi per liter at a rate of 2 liters per day used up half of the recommended maximum intake.

**Notes:**
In 1991, EPA proposed that the MCL for combined Ra-226 and Ra-228 be raised to individual Ra-226 and Ra-228 MCLs of 20 pCi/L.  The rationale was that water systems should focus their resources on radon exposure reduction since radium risks were estimated to be much lower.  Based on risk estimates at the time, individual Ra-226 and Ra-228 MCLs of 20 pCi/L would preserve the risk ceiling of one in ten thousand.  Gross alpha was proposed to remain at 15 pCi/L, but Ra-226 was to be excluded in addition to uranium and radon.  The beta and photon emitters MCL was to be re-defined in terms of an effective equivalent dose equivalent, which would have raised the individual radionuclide-specific concentration limits considerably.  Based on RADRISK, the computer risk model in use at the time, the one in ten thousand risk ceiling would have been preserved for an MCL of 4 mrem-ede per year.  The proposal also included an extension of the NPDWRs to NTNCWSs and point of entry monitoring.  It was reasoned that the current practice of distribution system monitoring would not be protective of customers that were served primarily by an entry point that had higher contaminant levels than the water in the distribution system.

## Radium MCL (cont.)

**1991 MCL**Ra-226 and Ra-228 = 20 pCi/L each
- Critical organ: Bone, bone marrow, nasal sinuses (Ra-226)
- Based on EPA’s RADRISK calculations
- Est. lifetime fatal cancer risk: 1 to 2 x 10-4

**Notes:**
In 1991, EPA proposed that the MCL for combined Ra-226 and Ra-228 be raised to individual Ra-226 and Ra-228 MCLs of 20 pCi/L.  The rationale was that water systems should focus their resources on radon exposure reduction since radium risks were estimated to be much lower.  Based on risk estimates at the time, individual Ra-226 and Ra-228 MCLs of 20 pCi/L would preserve the risk ceiling of one in ten thousand.  Gross alpha was proposed to remain at 15 pCi/L, but Ra-226 was to be excluded in addition to uranium and radon.  The beta and photon emitters MCL was to be re-defined in terms of an effective equivalent dose equivalent, which would have raised the individual radionuclide-specific concentration limits considerably.  Based on RADRISK, the computer risk model in use at the time, the one in ten thousand risk ceiling would have been preserved for an MCL of 4 mrem-ede per year.  The proposal also included an extension of the NPDWRs to NTNCWSs and point of entry monitoring.  It was reasoned that the current practice of distribution system monitoring would not be protective of customers that were served primarily by an entry point that had higher contaminant levels than the water in the distribution system.

## Radium MCL (cont.)

- *Lifetime excess cancers calculated using FGR-13 risk coefficients.

**Current Risk Estimates for Radium MCLs***

**Notes:**
In 1991, EPA proposed that the MCL for combined Ra-226 and Ra-228 be raised to individual Ra-226 and Ra-228 MCLs of 20 pCi/L.  The rationale was that water systems should focus their resources on radon exposure reduction since radium risks were estimated to be much lower.  Based on risk estimates at the time, individual Ra-226 and Ra-228 MCLs of 20 pCi/L would preserve the risk ceiling of one in ten thousand.  Gross alpha was proposed to remain at 15 pCi/L, but Ra-226 was to be excluded in addition to uranium and radon.  The beta and photon emitters MCL was to be re-defined in terms of an effective equivalent dose equivalent, which would have raised the individual radionuclide-specific concentration limits considerably.  Based on RADRISK, the computer risk model in use at the time, the one in ten thousand risk ceiling would have been preserved for an MCL of 4 mrem-ede per year.  The proposal also included an extension of the NPDWRs to NTNCWSs and point of entry monitoring.  It was reasoned that the current practice of distribution system monitoring would not be protective of customers that were served primarily by an entry point that had higher contaminant levels than the water in the distribution system.

## Health Basis for Alpha MCL

**1976 MCL**Gross alpha = 15 pCi/L, including Ra-226, excluding uranium and radon
- "Based on conservative assumption that if the radium concentration is 5 pCi/L and the balance of the alpha particle activity [10 pCi/L] is due to the next most radiotoxic alpha particle emitting chain, starting with lead-210 [i.e., Po-210], the total dose to bone would be equivalent to less than 6 pCi/L of radium-226.”
- Est. lifetime fatal cancer risk: 1 to 2 x 10-4 (or less)

**Notes:**
In 1991, EPA proposed that the MCL for combined Ra-226 and Ra-228 be raised to individual Ra-226 and Ra-228 MCLs of 20 pCi/L.  The rationale was that water systems should focus their resources on radon exposure reduction since radium risks were estimated to be much lower.  Based on risk estimates at the time, individual Ra-226 and Ra-228 MCLs of 20 pCi/L would preserve the risk ceiling of one in ten thousand.  Gross alpha was proposed to remain at 15 pCi/L, but Ra-226 was to be excluded in addition to uranium and radon.  The beta and photon emitters MCL was to be re-defined in terms of an effective equivalent dose equivalent, which would have raised the individual radionuclide-specific concentration limits considerably.  Based on RADRISK, the computer risk model in use at the time, the one in ten thousand risk ceiling would have been preserved for an MCL of 4 mrem-ede per year.  The proposal also included an extension of the NPDWRs to NTNCWSs and point of entry monitoring.  It was reasoned that the current practice of distribution system monitoring would not be protective of customers that were served primarily by an entry point that had higher contaminant levels than the water in the distribution system.

## Basis for Alpha MCL (cont.)

**1991 MCL**Adjusted gross alpha = 15 pCi/L,excluding Ra-226, uranium and radon
- Po-210 would be the most likely and most toxic alpha particle emitter in water, excluding Ra-226, uranium and Rn-222.
- Lifetime consumption of 14 pCi/L Po-210 was estimated to yield a 1 x 10-4 fatal cancer risk
- Est. lifetime fatal cancer risk: 1 x 10-4 (or less)

**Notes:**
In 1991, EPA proposed that the MCL for combined Ra-226 and Ra-228 be raised to individual Ra-226 and Ra-228 MCLs of 20 pCi/L.  The rationale was that water systems should focus their resources on radon exposure reduction since radium risks were estimated to be much lower.  Based on risk estimates at the time, individual Ra-226 and Ra-228 MCLs of 20 pCi/L would preserve the risk ceiling of one in ten thousand.  Gross alpha was proposed to remain at 15 pCi/L, but Ra-226 was to be excluded in addition to uranium and radon.  The beta and photon emitters MCL was to be re-defined in terms of an effective equivalent dose equivalent, which would have raised the individual radionuclide-specific concentration limits considerably.  Based on RADRISK, the computer risk model in use at the time, the one in ten thousand risk ceiling would have been preserved for an MCL of 4 mrem-ede per year.  The proposal also included an extension of the NPDWRs to NTNCWSs and point of entry monitoring.  It was reasoned that the current practice of distribution system monitoring would not be protective of customers that were served primarily by an entry point that had higher contaminant levels than the water in the distribution system.

## Basis for Alpha MCL (cont.)

**Current Risk Estimates for Alpha MCLs***

- *Lifetime excess cancers calculated using FGR-13 risk coefficients.

**Notes:**
In 1991, EPA proposed that the MCL for combined Ra-226 and Ra-228 be raised to individual Ra-226 and Ra-228 MCLs of 20 pCi/L.  The rationale was that water systems should focus their resources on radon exposure reduction since radium risks were estimated to be much lower.  Based on risk estimates at the time, individual Ra-226 and Ra-228 MCLs of 20 pCi/L would preserve the risk ceiling of one in ten thousand.  Gross alpha was proposed to remain at 15 pCi/L, but Ra-226 was to be excluded in addition to uranium and radon.  The beta and photon emitters MCL was to be re-defined in terms of an effective equivalent dose equivalent, which would have raised the individual radionuclide-specific concentration limits considerably.  Based on RADRISK, the computer risk model in use at the time, the one in ten thousand risk ceiling would have been preserved for an MCL of 4 mrem-ede per year.  The proposal also included an extension of the NPDWRs to NTNCWSs and point of entry monitoring.  It was reasoned that the current practice of distribution system monitoring would not be protective of customers that were served primarily by an entry point that had higher contaminant levels than the water in the distribution system.

## Health Basis for Beta MCL

**1976 MCL**4 mrem/y total body or critical organ dose
- Except for H-3 and Sr-90, derived activity concentrations each  beta/photon emitter calculated using the168-hr MPCw values from NBS Handbook 69 (ICRP Publication 2 (1959)) adjusted by a factor for exposure of the general population
- Sum-of-the-fractions method required for multiple, co-occurring beta/photon emitters
- Est. lifetime fatal cancer risk: 5.6 x 10-5 (or less)

**Notes:**
In 1991, EPA proposed that the MCL for combined Ra-226 and Ra-228 be raised to individual Ra-226 and Ra-228 MCLs of 20 pCi/L.  The rationale was that water systems should focus their resources on radon exposure reduction since radium risks were estimated to be much lower.  Based on risk estimates at the time, individual Ra-226 and Ra-228 MCLs of 20 pCi/L would preserve the risk ceiling of one in ten thousand.  Gross alpha was proposed to remain at 15 pCi/L, but Ra-226 was to be excluded in addition to uranium and radon.  The beta and photon emitters MCL was to be re-defined in terms of an effective equivalent dose equivalent, which would have raised the individual radionuclide-specific concentration limits considerably.  Based on RADRISK, the computer risk model in use at the time, the one in ten thousand risk ceiling would have been preserved for an MCL of 4 mrem-ede per year.  The proposal also included an extension of the NPDWRs to NTNCWSs and point of entry monitoring.  It was reasoned that the current practice of distribution system monitoring would not be protective of customers that were served primarily by an entry point that had higher contaminant levels than the water in the distribution system.

## Basis for Beta MCL (cont.)

**1991 MCL**4 mrem/y effective dose equivalent (ede)
- Derived activity concentrations each beta/photon emitter calculated using EPA’s RADRISK code (ICRP-30)
- Sum-of-the-fractions method required for multiple, co-occurring beta/photon emitters
- Est. lifetime fatal cancer risk: 1 x 10-4 (or less)

**Notes:**
In 1991, EPA proposed that the MCL for combined Ra-226 and Ra-228 be raised to individual Ra-226 and Ra-228 MCLs of 20 pCi/L.  The rationale was that water systems should focus their resources on radon exposure reduction since radium risks were estimated to be much lower.  Based on risk estimates at the time, individual Ra-226 and Ra-228 MCLs of 20 pCi/L would preserve the risk ceiling of one in ten thousand.  Gross alpha was proposed to remain at 15 pCi/L, but Ra-226 was to be excluded in addition to uranium and radon.  The beta and photon emitters MCL was to be re-defined in terms of an effective equivalent dose equivalent, which would have raised the individual radionuclide-specific concentration limits considerably.  Based on RADRISK, the computer risk model in use at the time, the one in ten thousand risk ceiling would have been preserved for an MCL of 4 mrem-ede per year.  The proposal also included an extension of the NPDWRs to NTNCWSs and point of entry monitoring.  It was reasoned that the current practice of distribution system monitoring would not be protective of customers that were served primarily by an entry point that had higher contaminant levels than the water in the distribution system.

## Basis for Beta MCL (cont.)

**Current Risk Estimates for Beta/Photon MCLs***

- *Lifetime excess cancers calculated using FGR-13 risk coefficients.

**Notes:**
In 1991, EPA proposed that the MCL for combined Ra-226 and Ra-228 be raised to individual Ra-226 and Ra-228 MCLs of 20 pCi/L.  The rationale was that water systems should focus their resources on radon exposure reduction since radium risks were estimated to be much lower.  Based on risk estimates at the time, individual Ra-226 and Ra-228 MCLs of 20 pCi/L would preserve the risk ceiling of one in ten thousand.  Gross alpha was proposed to remain at 15 pCi/L, but Ra-226 was to be excluded in addition to uranium and radon.  The beta and photon emitters MCL was to be re-defined in terms of an effective equivalent dose equivalent, which would have raised the individual radionuclide-specific concentration limits considerably.  Based on RADRISK, the computer risk model in use at the time, the one in ten thousand risk ceiling would have been preserved for an MCL of 4 mrem-ede per year.  The proposal also included an extension of the NPDWRs to NTNCWSs and point of entry monitoring.  It was reasoned that the current practice of distribution system monitoring would not be protective of customers that were served primarily by an entry point that had higher contaminant levels than the water in the distribution system.

## Basis for Beta MCL (cont.)

**Current Risk Estimates for Beta/Photon MCLs***

- *Lifetime excess cancers calculated using FGR-13 risk coefficients.

**Notes:**
In 1991, EPA proposed that the MCL for combined Ra-226 and Ra-228 be raised to individual Ra-226 and Ra-228 MCLs of 20 pCi/L.  The rationale was that water systems should focus their resources on radon exposure reduction since radium risks were estimated to be much lower.  Based on risk estimates at the time, individual Ra-226 and Ra-228 MCLs of 20 pCi/L would preserve the risk ceiling of one in ten thousand.  Gross alpha was proposed to remain at 15 pCi/L, but Ra-226 was to be excluded in addition to uranium and radon.  The beta and photon emitters MCL was to be re-defined in terms of an effective equivalent dose equivalent, which would have raised the individual radionuclide-specific concentration limits considerably.  Based on RADRISK, the computer risk model in use at the time, the one in ten thousand risk ceiling would have been preserved for an MCL of 4 mrem-ede per year.  The proposal also included an extension of the NPDWRs to NTNCWSs and point of entry monitoring.  It was reasoned that the current practice of distribution system monitoring would not be protective of customers that were served primarily by an entry point that had higher contaminant levels than the water in the distribution system.

## Health Basis for Uranium MCL

**1976 MCL**Not regulated
**1991 MCL**20 micrograms/L (~30 pC/L)
- based on chemical toxicity in kidney, assuming a 1000-fold uncertainty factor and Reference dose (RfD) based on short-term animal studies
- 30 pCi/L equivalent to 20 micrograms/L assuming ratio of 1.3U-234:U-238 activity from NIRS database)
- Est. lifetime fatal cancer risk: 1 x 10-4

**Notes:**
In 1991, EPA proposed that the MCL for combined Ra-226 and Ra-228 be raised to individual Ra-226 and Ra-228 MCLs of 20 pCi/L.  The rationale was that water systems should focus their resources on radon exposure reduction since radium risks were estimated to be much lower.  Based on risk estimates at the time, individual Ra-226 and Ra-228 MCLs of 20 pCi/L would preserve the risk ceiling of one in ten thousand.  Gross alpha was proposed to remain at 15 pCi/L, but Ra-226 was to be excluded in addition to uranium and radon.  The beta and photon emitters MCL was to be re-defined in terms of an effective equivalent dose equivalent, which would have raised the individual radionuclide-specific concentration limits considerably.  Based on RADRISK, the computer risk model in use at the time, the one in ten thousand risk ceiling would have been preserved for an MCL of 4 mrem-ede per year.  The proposal also included an extension of the NPDWRs to NTNCWSs and point of entry monitoring.  It was reasoned that the current practice of distribution system monitoring would not be protective of customers that were served primarily by an entry point that had higher contaminant levels than the water in the distribution system.

## Basis for Uranium MCL (cont.)

**Current Risk Estimates for Uranium MCL***

- *Lifetime excess cancers calculated using FGR-13 risk coefficients.

**Notes:**
In 1991, EPA proposed that the MCL for combined Ra-226 and Ra-228 be raised to individual Ra-226 and Ra-228 MCLs of 20 pCi/L.  The rationale was that water systems should focus their resources on radon exposure reduction since radium risks were estimated to be much lower.  Based on risk estimates at the time, individual Ra-226 and Ra-228 MCLs of 20 pCi/L would preserve the risk ceiling of one in ten thousand.  Gross alpha was proposed to remain at 15 pCi/L, but Ra-226 was to be excluded in addition to uranium and radon.  The beta and photon emitters MCL was to be re-defined in terms of an effective equivalent dose equivalent, which would have raised the individual radionuclide-specific concentration limits considerably.  Based on RADRISK, the computer risk model in use at the time, the one in ten thousand risk ceiling would have been preserved for an MCL of 4 mrem-ede per year.  The proposal also included an extension of the NPDWRs to NTNCWSs and point of entry monitoring.  It was reasoned that the current practice of distribution system monitoring would not be protective of customers that were served primarily by an entry point that had higher contaminant levels than the water in the distribution system.