TOPIC 6: Types of Risks

Let’s explore type of RISKS.

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What is risk.  How often do we take risks?  We take risks from the time we crawl out of bed until we crawl back into bed.  But then again, we take risks while we sleep.  Tornadoes, trees falling on your house.  There is just no way to avoid risks.

Here is another way of looking at risk. Instead of looking at things that have different risks, let’s fix the risk at 1 in a million and see what things cause that level of risk.

Lets take for instance “smoking 1.4 cigarettes” leads to “cancer, heart disease.”  What that means is that for every 1.4 cigarettes smoked, 1 out of 1 million people will develop cancer or heart disease. There are some events on the list that you wouldn’t think twice about doing, so,  maybe 2 mrem whole-body dose isn’t that bad in terms of risk.

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About half of all US citizens get cancer in their lifetimes.  Really!  Most people don’t realize that the cancer incidence is so high. The most common is prostate cancer for men and breast cancer for women.  Even with improvements in medicine and treatments, still at least 20% of US citizens die from cancer.   As the average age has increased over the last 50 years, the incidence of cancer has increased also.   But, it is hoped that this value will decrease as research provides more answers to what causes cancer and how to treat it.

Here are the increased risks of cancer mortality from 100 mrem of radiation:

–Solid tumor cancer risk is about one chance out of 25,000 (1:25,000)

–Leukemia risk is about one chance out of 125,000 (1:125,000)

This equates to an overall 0.005% increased risk of cancer mortality from a 100 mrem exposure to radiation.  This is extremely small compared to approximately 55% total cancer risk from everything else on this planet and >20% cancer mortality to our population from these other risks.

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We interpret this information as 1-colon-4 means a one in four chance. That is really high. The average lifetime smoker is really rolling the dice. They have a 25% chance of dying from that habit alone. Interestingly, people develop cancers whether they are exposed to carcinogens or not. We still find lung cancer amount non-smokers.  But we find many more lung cancers among smokers – with the incidence of lung cancer higher among the heavier smokers. But even heavy smoking doesn’t guarantee that the smoker will get cancer. They are simply at a higher risk.

We all recognize that police work is dangerous, but what about working in agriculture. Farmers and ranchers work with chemicals like herbicides and pesticides, as well as dangerous heavy machinery.

We don’t give much thought to the danger of driving to work every day or taking a road trip for pleasure. The average risk is about 1 in 6,000 per year of death while driving. So you think that the fatal car accident won’t happen to you. But yet you buy a lotter ticket that has a chance of 1 in 300,000,000 because you feel you can beat the odds!

Notice that the annual risk of death from falls is 1 in 20,000. Most of those falls take place at home as opposed to a mountain. The average person also has a 1 in 50,000 of dying in a home fire. There are plenty of other risks you face at home, like poisoning and electrocution. But home is actually a pretty safe place to be, and we tend to be at ease there.

It’s no accident that the annual public dose limit is set at 100 mrem. As you have now seen, the risk from 100 mrem is in line with the risk of being at home, which is about 1 in 20,000.

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If we plot risk on a logarithmic scale – where every major division in the scale is ten times the previous one – we can see risks from low to high and establish a “safe zone.” That is where we want to keep doses to the public. Once plotted, we can visually determine what is safe.

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Here are risks from simply living at home compared to a Member of the Public dose to radiation of 100 mrem per year.  Member of Public, or MOP, is a new term we will explore later.

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If a tree falls in the forest and no one hears it, does it still make a sound?  Likewise, if one cannot see the effects of radiation, are they really there? Does the smallest dose of radiation increase the risk of getting a fatal cancer? Is there some level below which there is no risk from radiation? Could this low dose of radiation actually be beneficial?

These questions have caused controversy among scientists for years. Scientists know from accidents involving radioactive material that a high acute radiation dose causes cellular damage. There are definite thresholds associated with these acute effects. They also know that as the dose increases, the severity of the injury also increases. Scientists also know that there are no visible effects at low doses, and there is no way to distinguish the risk of developing a fatal cancer at these low doses. There has been no indisputable proof one way or the other that a dose always has a risk, which creates an argument.

Everyone agrees there is damage at high doses.  But there is a big controversy about what happens at low doses.

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The Linear Non-Threshold (LNT) Model has general consensus among experts and is used for radiation protection purposes because it very likely overestimates risk at low doses. This model is accepted by regulatory agencies since it is the most conservative model, which makes it useful for establishing acceptable dose limits.  This model is not biologically realistic from an understanding of dose and dose response as there are no observable effects at low doses, but it provides an extra degree of safety when regulating the industry and protecting workers and the public.

The model is linear because the dose-response curve is a straight line. Any increase in dose results in a proportional increase in risk. It is non-threshold because there is no threshold dose below which no response (increase in risk) is allowed. Any dose, no matter how small, is assumed to produce some risk. It is impossible to prove or disprove this assumption. You could apply this assumption to any health hazard. For example, if drinking an excessive amount of water in a short time period can kill you (and it can!), then there is a risk of dying with every sip of water you ever drink. We know that this is stretching the truth, but when this assumption was applied to radiation, laymen accepted it as truth rather than an unproven assumption only adopted to keep radiation workers safe.

The LNT model also ignores the fact that the body has an excellent ability not only to repair itself, but also to protect itself from any problems that arise. Every person would eventually develop cancer if the body’s defenses couldn’t kill individual cancer cells, because we all have hundreds of pre-cancerous cells in our bodies at any given time.

The only point of zero occupational risk is at zero occupational dose, which is where the line begins at the left. In reality, there is always a dose from something, but there isn’t always a dose from a licensed source. The slope of the line is at a 45 degree angle, which means the doses and responses are proportional throughout the chart. A person who receives 5 rem equivalent dose in a year incurs 10 times as much risk as a person who receives 0.5 rem in a year.  The model assumes that more dose means a proportional increase in risk.

It has been known for a long time that radiation can increase the incidence of cancer at high acute doses (above 10 rem or 0.1 Sv acute dose), but nobody has ever known if low doses are not harmful or if there is some threshold level where the risk begins. In fact, the health risk from small radiation doses is so small that no scientific research methods are able to distinguish it from zero. In the 1950s, when faced with such unknowns, a group of prominent radiation protection specialists met in the U.S. to adopt a philosophy that would be most protective of human health.  They decided to use a very protective risk model that follows the assumption that every increase in dose imparts a proportional increase in risk.  The known risk at high doses that had been established through real data was extrapolated to the low dose region where no reliable data are available. This is the linear non-threshold (LNT) model still used over six decades later. The bottom line is that the LNT model was never meant to describe the actual biological situation, but rather was meant only to be a conservative model for radiation protection considering the current state of scientific uncertainty.

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This chart indicates the region of the LNT model where environmental doses dominate. Some well-respected scientists have argued that we should not regulate in this dose region (up to about 800 mrem), but even they cannot overcome over six decades of regulatory doctrine.

If every increment of radiation dose increases risk and therefore increases the chance of cancer, then you would expect to see higher cancer incidences in areas with higher natural, or background, radiation. But this is not the case.  The seven western states in the U.S. with the highest natural background radiation have cancer mortalities 15% lower than the other states.  One might be tempted to draw the conclusion that the extra radiation is healthful, but once again there are too many variables and too much uncertainty.  It is quite possible that people in those states are simply exposed to fewer carcinogens in general. We will develop this further when we discuss the hormesis model.

Biological dose-response studies suggest that cancer, or a genetic change, can be initiated by improper repair within a single DNA molecule. The same basic molecular event, that produces both cancers and genetic effects, is more often the result of chemical agents than radiation.

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The linear-quadratic (or non-threshold) model is closer to biological reality because it takes into consideration the biological repair mechanisms that do occur.  Instead of a straight line, there is a curve so that there is a slight risk at low doses , but not nearly as much as the linear LNT model.

Empirical data from animal studies has been observed to follow this model. Note that there is neither a threshold below which there is no risk, and there is no benefit from low doses of radiation.

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Linear with Threshold Model

The threshold theory maintains no increase in risk with dose until a certain point is reached in the low dose realm, and then there is a linear dose-risk response.

After the threshold, this linear with threshold model is similar to the linear part of the LNT model in other aspects, with the exception of the angle of the slope of the line.

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The supra-linear model predicts greater harm at low doses. This assumption is difficult to justify because biological repair mechanisms evolved over millions of years in a low dose environment, and we know that those mechanisms are efficient.

This model has some studies in its favor, but few supporters.

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Some pioneers in the radiation science in the past have affirmed that low levels of radiation have no detrimental effects and may have a hormetic or beneficial effect.   The term “hormesis” is derived from “hormaein” which means “to excite.”  Generally, hormesis is a beneficial or stimulating effect induced by low doses from an agent, whereby the potential detrimental effects at linearly higher doses from the same agent cannot be predicted.  There is some biological evidence in other species of plants and animals that supports this model. However, with the lack of human data at low doses, this model is not popular with regulators. If this model were to replace the LNT model, there would be no need for most environmental regulation of radioactivity, dose limits would change, and some doses may be considered as having negligible risk. Well, there recently was a strong petition to replace the LNT with another model, and the hormesis model was the one of choice, but it was effectively stalled.

This graph shows that the risk which is considered to be a potential negative to the body goes below zero meaning there is a positive benefit at low doses.

There have been thousands of studies claiming that the body cells are stimulated by low doses of radiation.  In the late 1900s, some studies in mice showed that an increased dose from X-rays reduced cancer incidence.   Also, it claimed that mice given lethal doses of radiation had a better chance of survival if they had receive some prior dose.

Studies in India stated that those persons living in a higher background radiation area had fewer cancers.  In another study, nuclear power plant workers in Canada were shown to have a 58% less chance of getting cancer than non-nuclear power plant workers. Such studies indicate possible hermetic effects, but are not scientifically robust enough to be definitive.

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The most remarkable aspect of hormesis is that low doses of harmful agents can produce the effects that are diametrically opposite to effects seen with high doses of the same agents.  Such effects as accelerated germination, sprouting, growth, development, blooming, ripening, increased crop yield and resistance to disease are found in plants irradiated with low doses of ionizing radiation.

There was a 100-year study of British radiologists who worked from1897-1920 and received an absorbed dose estimated at about 100 rad per year. For non-cancer causes of death there was no evidence of any increased risk.  These early doctors lived significantly longer than other male doctors. Their cancer death rate was significantly lower than that of all men in England and Wales.  After 1955, the death rate was lower than other medical disciplines.  Lauriston Taylor, one of the founders of the International Commission on Radiological Protection, wrote in 1980 “the theories about people being injured have still not led to the demonstration of injury and, if considered as facts by some, must only be looked upon as figments of the imagination.”  The British radiologist study will not resolve the controversy concerning the validity of the linear non-threshold (LNT) model of radiation risk, but it casts doubt on the assumption that low levels of radiation have no beneficial effect on humans.

We do not know what causes these hermetic changes; but, there are several theories.  At the molecular level, low doses of radiation create certain types of proteins that enable the DNA to more readily repair itself.  Another study indicated that radiation inhibited the DNA synthesis, which allowed more time for the irradiated cells to recover.  In the early 1900s, studies showed that mice treated with low levels of radiation developed a resistance against bacterial diseases.  Later in the century, studies supported the immune-stimulatory effects of low doses of ionizing radiation.  The concept of hormesis remains an intriguing mystery and will continue to be studied well into the future.

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• A good example of non-ionizing radiation hormesis is UV in sunlight. At low doses it is beneficial by producing vitamin D that is vital for our health, but at high levels it is dangerous and produces burns and induces skin cancer.

 

• Caffeine and alcohol have mild and desirable effects at low doses, but at high doses are detrimental or even lethal.

 

• Two or three aspirin can help to ease pain and inflammation and is therefore beneficial, while an entire container of aspirin taken at once could cause death.

 

• Put a little fertilizer on the lawn and it grows green and healthy. Put a lot of fertilizer on the lawn and it burns and dies.

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As we have discussed, there are a handful of competing models of dose effects. But there are two significant scientific camps in the battle. On the one side of this argument are those that support the Linear Non-Threshold (LNT) model.  This model simply states: since large amounts of radiation cause death, any exposure to radiation must cause some damage and some cancer risk.  In essence, there is no threshold dose below which radiation exposure would be safe. This is also the model used by regulators today.

The other model is hormesis.  It basically states that there is a benefit for radiation exposure at low doses.

The only thing that you can say with certainty is that the health risk from small radiation doses is so low that no scientific research methods are able to distinguish it from zero. In the very high dose region, there is much reliable data from bomb detonations and severe accidents involving nuclear material. There is no doubt, at these levels, mortality risk increases with increased radiation doses.

Summary

It is much easier to test the dose-response relationships on plants than animals and definitely not humans. To make the assumption that a particular model applies to man when the model basis used plants or small mammals as the test subjects is somewhat presumptuous. So, should we make the assumption that it would also apply to man? Well, since we don’t know, and making far-reaching assumptions would be less than scientific, the most conservative model, the LNT model, is adopted for regulatory controls.