Table of Contents
Implications of the Hormesis Hypothesis for Risk Perception and Communication

Ortwin Renn, Ph.D.

Center of Technology Assessment,

Industriestrasse 5, 70565 Stuttgart, Germany,

Phone[49] (711) 9063 160,

Fax [49] (711) 9063 299


There has been a vigorous debate in the scientific community about the validity of the hormesis hypothesis. Stebbing defines hormesis as the stimulatory effects caused by low levels of toxic agents (Stebbing 1982). Toxic agents that are detrimental to human health above certain threshold levels may induce positive effects at a dose that is significantly lower than the NOAEL level (Calabrese and Baldwin 1997). Such positive stimulation has not been the focus of regulatory activities since regulatory agencies define their mandate to mean protection of the public from health risks and not an endorsement of positive health effects. Since the existence of hormesis is not easily demonstrated and since hormesis contradicts the commonly used linear dose-response models, regulatory agencies prefer to ignore this phenomenon as not yet proven or to deem it irrelevant for pursuing their public mandate. "To be on the safe side" is the main argument for treating toxic agents as purely negative causes for health detriments.

The picture becomes even more complicated when carcinogenic agents are included (Teeguarden, Dragan and Pilot 1998). Most dose-response models assume a finite probability for developing cancer as a result of any exposure above zero. These stochastic effects are due to the possibility of irreversible damage to the DNA at an exposure level as low as a single molecule. If the hormesis hypothesis would also hold true for carcinogenic substances or radiation, the probability for a tumor inhibition may outweigh the probability of a tumor induction. Under these circumstances the situation might occur that a single individual may develop a tumor as a result of an exposure to a very small dose of a carcinogen, while the majority of people may experience positive inhibitory effects. Similar dilemmas are to be expected even with simple toxic substances if individuals vary in their sensibility towards the beneficial range of exposure in which the positive effects are observed. One individual may experience the positive effects at a different dose range compared to another more sensitive individual. How should a regulator evaluate such a situation? Is it justified to endorse exposure to small concentrations of a proven carcinogenic or toxic substance if there is a chance that a small number of people will probably be negatively affected while the majority enjoys the potential benefits? This question raises equity concerns and leads to difficult policy dilemmas. Furthermore, as studies on alcohol seem to suggest, advising people to drink moderate amounts of alcohol are likely to be misunderstood by many as a legitimation to continue their unhealthy drinking habits while others who are at risks to become alcoholics might reduce their resistance to alcohol and give up their abstinence. The policy question here is to make sure that not more than the beneficial amount is consumed although the toxic agent may lead to addiction

According to my knowledge, the implications of hormesis for public policy making and risk communication have not been adequately addressed in the social science literature to this date. Indeed it may be a reasonable strategy to resolve the major remaining problems with hormesis within the risk expert community before demanding wide-reaching changes in regulatory practices or policies. Living in a pluralist information society, however, implies that major scientific controversies will find their way into the public debate before the issues are resolved in the scientific community. For this reason, it may be valuable to analyze the potential public reactions to the hormesis hypothesis and to draw some inferences about the likely policy changes that are needed if the hormesis hypothesis becomes an established "fact" within the scientific community. Empirical studies on the perception of hormesis are hence needed; so far one can only draw analogies to similar situations, such as the perception of instructions on drug use, the evaluation of homeopathic doses, or the insights from psychosomatic investigations. To pick up one analogy, there has been some treatment in the literature on the role of public perception with respect to fluoridation of drinking water (Mazur 1975; 1981). Most of the public resistance was explained there as a statement of opposition to state interference in personal freedom, less, though, as a result of a disbelief in its positive health outcomes. This may be quite different if well-known toxic substances or radiation is taken as an example for hormetic effects.

This paper provides a brief review of the present knowledge of risk perception and its application to risk communication. Based on these insights, the major implications of hormesis for shaping risk perceptions are discussed. At the end the paper lists several recommendations for risk communication under the assumption that the hormesis hypothesis can be further substantiated.


Attention and selection filters

Today's society provides an abundance of information, much more than any individual can digest (Renn 1997). Most information to which the average person is exposed will be ignored. This is not a malicious act but a sheer necessity in order to reduce the amount of information a person can process in a given time. The attention and selection process is not random although random elements may play a role. People have developed special strategies to select information that they feel is relevant to them. This is also true for risk information. The major criteria for selection are ability and motivation (Chaiken and Stangor 1987). Ability refers to the physical possibility that the receiver can follow the message without distraction, motivation to the readiness and interest of the receiver to process the message.

If both criteria are met, the individual selects the information and processes it further. However to economize further on time, s/he is going to evaluate whether it is necessary to study content of the information in detail or to make a fast judgment according to some salient cues in the message received. The first strategy refers to the central route of information processing, the second to the peripheral route (Petty and Cacioppo 1986; Renn and Levine 1991). The central route is taken when the receiver is so highly motivated by the message that s/he studies each argument carefully. The peripheral route is taken when the receiver is less inclined to deal with each argument, but forms an opinion or even an attitude on the basis of simple cues and heuristics. Communicating with the first group demands thorough argumentation and detailed analysis of decision options (which usually deters the peripherally interested persons); communicating with the second group requires the use of entertaining and appealing cues such as colored brochures and trust-building Public Relations (which unfortunately alienates the centrally interested group). A good risk communicator should try to identify the type of audience (central or peripheral) before designing the communication process.

Applying these insights on selection procedures to information on hormesis, it is quite obvious that at this stage of the debate, the implications for public policy have not yet passed the selection filter of most people. Except for a small group of experts, most policy makers, stakeholders and the public at large are not yet aware of the "flip-flop effect" that goes along with different levels of exposure. Although anecdotal knowledge suggests that small amounts of a "bad agent" might be beneficial as experienced with many medical drugs or even alcohol, it is rather unlikely that such effects are associated with "artificial" chemicals in the food chain or environmental pollutants. Even if the information would be made accessible to the public, most people would probably judge this information as not very credible.

The main reason for the rejection would be the fact that chemicals in food and water as well as environmental pollutants are commonly regarded as negative outcomes of an industrialized society (Slovic et al. 1981; Slovic 1992; Tanaka 1991). People believe that these chemicals are the price we have to pay for the benefits of living in an mostly affluent society. There is a wide disagreement among stakeholders and members of the public whether this price is too high or whether it is justified compared to the benefits, but everybody shares the notion that any substance released into the air or water that is not part of the desired product has a negative impact on the quality of human health and the environment. How can something be beneficial that is widely perceived as a powerful symbol pointing towards the dark side of industry and profit-seeking enterprises? Is this another trick of industry to avoid costly risk reduction and mitigation measures?

Obviously the debate on hormesis will gain public attention and probably motivate people to reflect on the meaning of toxicity with respect to substances as well as lifestyles. The centrally interested audience will collect information pro and con while the peripherally interested are keen on easily available clues for orientation. It is too early to make any predictions on what the resulting judgment will be, but I am certain that the hormesis hypothesis will lead to a heated debate among policy makers, stakeholders and the public at large since it challenges conventional wisdom and widely accepted political and regulatory routines.

Intuitive heuristics

Once information has been received, common sense mechanisms process the information and help the receiver to draw inferences. These processes are called intuitive heuristics. They are particularly important for risk perception, since they relate to the mechanisms of processing probabilistic information. One example of an intuitive strategy to evaluate risks is to use the mini-max rule for making decisions. This rule implies that people try to minimize post decisional regret by choosing the option that has the least potential for a disaster regardless of probabilities. The use of this rule is not irrational (Lee 1981). It has evolved over a long evolution of human behavior as a fairly successful strategy to cope with uncertainty (better safe than sorry).

This heuristic rule of thumb is probably the most powerful factor for rejecting or downplaying information on hormesis. If any exposure above zero or above a defined threshold (minus safety factor) is regarded as negative, the simple and intuitively reasonable rule to minimize exposure makes perfect sense. Most regulatory regimes are based on this simple rule ranging from the ALARA principle to the application of the best available control technology (BACT). Such principles lose their justification if one expects a positive effect from exposure to a defined range in the chemicalŒs concentration. In this case, individuals would be well advised to seek an exposure that lies within the range of beneficial outcomes. The task of determining such a range is further complicated by the fact that there may be considerable inter-individual variances in susceptibility and that stochastic effects may be in place that allow only probabilistic inferences.

Most individuals would probably be simply overtaxed by taking all these factors into account. It is unlikely that each individual will be given the opportunity to explore his or her individual profile of susceptibility with respect to potentially beneficial agents. Regulatory actions can set limits but cannot force individuals to accept exposures that may be detrimental to some individuals while benefiting the vast majority. Either public health officials can define reasonable ranges of beneficial exposure that would affect almost everyone in a positive manner regardless of individual or social sensibility or continue to base regulatory actions only on the negative impacts of any given agent.

In addition to the "better safe than sorry" rule, risk perception researchers have identified other biases in people's ability to draw inferences from probabilistic information. These biases are summarized in Table 1 (Kahneman and Tversky 1979; Ross 1977; Renn 1990).

Risk managers and public health professionals should be aware of these biases because they are found in public perception and may be one of the underlying causes for the observed public response. For example, the frequent media coverage about leukemia and thyroid cancer cases based on exposure to radioactive substances has alarmed the public and promoted a response of outrage based on the availability bias (Drottz-Sjöberg and Persson 1993). As information on the negative impacts of radiation are widely disseminated in the media and any cancer case in the neighborhood of nuclear installations are associated with radiation, it is almost impossible to convey the message that small doses of ionizing radiation may actually inhibit cancer rather than induce it.

It may be interesting to note that until the late 1950s the German Spa of Bad Reichenhall advertised its water treatment facilities as a cure against many diseases with the argument the water would contain radon and its derivatives. With the advent of the nuclear power controversy, that argument has been dropped and the water treatment has been terminated. Radiation has been stigmatized as a cause of cancer (Slovic 1996). It will be extremely difficult to overcome this availability bias if indeed the positive effects of low dose radiation were proven true. Yet even this conditional statement is hard to meet, since the evidence for a potentially beneficial effect will never been proven beyond any reasonable doubt. Furthermore, the stochastic nature of the effect implies that at least one individual might be harmed even if most individuals experience health benefits. Radiation is certainly the most difficult case for any risk communication program that would be aimed to convey the good news of hormesis.

Semantic images

Psychological research has revealed different meanings of risk depending on the context in which the term is used. Whereas in the technical sciences the term risk denotes the probability of adverse effects, the everyday use of risk has different connotations. With respect to technological risk Table 2 illustrates the main semantic images (Renn 1989):

Agents that are likely to induce hormesis effects are mostly to be found in the category of PandorraŒs box. This has far-reaching implications. Most agents belonging to the Pandorra category are regarded as potentially harmful substances that defy human senses and "poison" people without them knowing about it. Risks associated with food additives, air pollutants, water impurities, and other chemical agents are mostly invisible to the person exposed and thus requires warning by regulators or scientists. In contrast to medical drugs where people are aware of the beneficial effects at the prescribed dose, additives or pollutants are never associated with intended positive impacts but always as negative side effects of an activity that provides some utility to society or groups of society. Along with that image people tend to believe that toxicity depends less on dose than on the characteristics of the substance (Kraus et al. 1992). Hence they demand a deterministic regulatory approach when it comes to controlling chemicals in the environment . Most surveys show that people demand zero-risk-levels, at least as the ideal target line (Sjöberg 1994). Risks belonging to the other categories are subject to deliberations between benefits and risks and lead to trade-offs. Risk within the Pandora Box, however, trigger feelings of avoidance and strict regulatory prohibitions (Renn 1989). The former US food regulations (the so called Delaney clause) reflect this public sentiment. Something that is regarded as truly bad and vicious is almost impossible to link with a positive connotation.

The only exception may be the exposure to "natural" agents (Rohrmann 1995). Most people believe that anything that exists in nature cannot be harmful for people if consumed in modest amounts. That is why alleged natural drugs are associated with fewer or even none negative side effects compared to alleged chemical drugs. The perceptions of natural toxins as benign reflect the modern impression or myth of "Mother Nature" who offers an invaluable set of beneficial resources to humankind in response for taking good care of Her. Chemical compounds, however, are associated with artificiality and seen as threats to human health. They are only beneficial for humans in exceptional situations such as in cases of severe illness. In all other applications they should be avoided or minimized as they disturb the purity of natural resources such as water or food. If any of these "chemical" compounds turn out to be beneficial if consumed in small amounts, it would contradict anything that most modern people believe in. It would turn the beliefs of the intuitive toxicologists upside down.

Qualitative risk characteristics

In addition to the images that are linked to different risk contexts, the type of risk involved and its situational characteristics shape individual risk estimations and evaluations (Slovic 1987). Psychometric methods have been employed to explore these qualitative characteristics of risks (Slovic et al. 1981; Vlek and Stallen 1981; Renn 1990; Gould et al. 1988; Rohrmann 1995). Surveys and experiments based on the psychometric paradigm have revealed that perception of risks is influenced by a series of perceived properties of the risk source or the risk situation. These properties are called qualitative characteristics. Table 3 lists the major qualitative characteristics and their influence on risk perception.

In addition to the qualitative factors listed in Table 3, equity issues play a major role in risk perception. The more risks are seen as unfair for the exposed population, the more they are judged as severe and unacceptable (Kasperson and Kasperson 1983). The perception of health risks induced by chemicals or radiation is usually linked to an absence of personal control and the preponderance of dread thus amplifying the impression of seriousness.

Furthermore, the perception of risk is often part of an attitude that a person holds about the cause of the risk, i.e. industrial activity, consumption of food, energy production and others. Attitudes encompass a series of beliefs about the nature, consequences, history, and justifiability of a risk cause (Otway and Thomas 1982; Sjöberg 1997). Due to the tendency to avoid cognitive dissonance, i.e. emotional stress caused by conflicting beliefs (Festinger 1957), most people are inclined to perceive risks as more serious and threatening if the other beliefs contain negative connotations and vice versa. Often risk perception is a product of these underlying beliefs rather than the cause for these beliefs (Clarke 1989).

With respect to the qualitative characteristics, one would expect that hormesis poses a serious challenge to long-standing beliefs. First, most chemicals are associated with negative risk characteristics such as dread, lack of personal control, and invisibility (Kraus et al. 1992; Tanaka 1991). These characteristics make people even more concerned about the negative impacts than warranted by the predicted health effects alone. Second, the beliefs associated with the risk source, for example industry, center around greed, profit-seeking and alleged disrespect for public health. The ongoing debate on the role of the tobacco industry in deceiving public opinion may be a good illustration of this negative image. The same impression dominates the perception of chemicals in the food or water chain. Fourth, the possibility of hormesis touches upon serious equity concerns if susceptibility to positive effects vary considerably among individuals or rest on probabilistic balancing. For all these reasons, it cannot be expected that the hormesis hypothesis will gain much public enthusiasm or support among most people although the message of hormesis provides some comfort to people who have been worried about minute amounts of chemical exposure. Who would not be happy if an exposure that was regarded as detrimental or at best indifferent to human health has actually positive health effects?

In the present social setting, however, even good news need to be compatible with what people believe is right or wrong and what is bad or good. Considering that the hormesis hypothesis will invoke continuous and controversial debate among toxicologists and other risk experts, the public will most likely respond with skepticism and disbelief. The only exception will be those agents that are perceived as natural.

Perceptions and psychosomatic responses

A major body of literature exists on how people assimilate information about hazards and transform them into somatic or psychological effects (Colligan et al 1982; Aurand et al. 1993). If a person feels threatened by a risk, stress and other somatic effects are likely to occur. Psychosomatic reactions can manifest themselves in two different forms: suppression of a real health threat and amplification of perceived health risks. In both cases, psychological factors govern the human response and may induce somatic reactions. It is well known that many patients respond with real improvements of clinical symptoms when exposed to placebos. On the other hand, many people feel threatened by environmental pollutants although dose-response-studies would not suggest any health effect. Table 4 provides an overview of the causes of stress, suppression of health impacts and amplification of such impacts (taken from Renn 1997).

The inducement of stress is likely to be linked to three major influences: real or perceived exposure to a risk agent, the perception of confusion about the risk level and appropriate reaction and finally the perception of insufficient time to make the necessary adjustments. If one of these factors is present, people tend to respond with increased concern and worry. If all these factors are present combined with symbolic connotations of environmental threats and the perception of technological hubris, a response as strong as observed in the aftermath of Chernobyl is not so surprising (Drottz-Sjöberg and Sjöberg 1990).

One of the most interesting questions surrounding the hormesis theses will be the question on the interaction between hormesis and psychosomatic reactions. In popular medicine, homeopathic treatment has gained much acceptance because it presupposes that extremely small concentrations of a natural substance can have large effects on improving human health. Most established physicians and biochemists attribute the success of homeopathy to placebo effects. They claim that people believe in the positive effects and activate their immune system to fight the disease. This article is not the place to discuss the arguments for or against homeopathy. But studying the perception of alternative medical treatments may provide some clues of how hormesis might be perceived in the future. Similar to the placebo effect that may explain therapeutical success, nocebo effects may result from fear and negative expectations with respect to small amounts of toxic agents. Diffuse syndromes such as the sick-house syndrome or the multiple-chemical-syndrome have often been associated with an interactive effect triggered by the combination of actual exposure and psychological distress and anxiety (Davidoff 1992). One can only speculate about the potential impacts of hormesis on psychosomatic reactions. If the hormesis hypothesis is believed, it may amplify the positive effect. If it is not believed or even seen as another "dirty" trick by industry to deceive the public, negative psychosomatic reactions are to be expected. People with strong environmentalist values would be enticed to make a point by showing that the promised positive effects did not occur and that they have become sicker than before.

Such individual case studies do not prove anything, but they exert strong repercussions on public opinion. If special advocates provide sufficient anecdotal evidence that they became sick after being exposed to a dose that proponents of the hormesis hypothesis would regard as beneficial most people would tend to reject the systematic claims and respond in the usual "better safe than sorry" mentality.


If the hormesis hypothesis is taken seriously, it would require a thorough revision of regulatory philosophy and actions (Sielken and Stevenson 1998) The minimization principle on which most of the regulations rests would be in need of either replacement or amendments. If public policy is meant to improve public health and not only to prevent negative effects, there would be a necessity to seek exposure to small concentrations or at least to ensure that such an exposure is not prohibited by the minimization principle. In the case of toxic substances with a clear NOAEL, only little changes in the regulatory system are required. Individuals may then be advised to seek exposure rather than avoid it as long as the NOAEL threshold (minus safety factor) is not reached.

The regulation of carcinogenic substances would require more substantial changes, however. If it is proven that carcinogenic agents have the potential to inhibit as well as induce cancer, new policies would be needed that provide legitimate and equitable trade-offs between individual cancer risk and public health benefits. The popular question "how safe is safe enough?" would not only need the addition of "how safe is fair enough?" but also "what degree of safety implies living less safe than possible?". The regulatory paradigm of minimization would need to be replaced by a new optimality rule that allows for beneficial effects of low dose exposure. Policy instruments for reaching this new paradigm are not yet in place and would require more deliberation and extended policy studies.

One of the most prominent obstacles towards implementing such a new policy would be risk perception. Although the hormesis hypothesis can be classified as "good" news and should be welcomed by most people potentially affected by exposure to small doses, it contradicts most of the salient beliefs that people have about chemicals in the environment. Regardless whether individuals prefer industrial or environmental values, their cognitive frame is marked by the concept of chemicals as pollutants and poisons (Kraus et al. 1992). The two groups differ in their evaluation of this cognitive "truth". The industrialists accept such pollution as a fair price to pay for all the benefits of the industrial society, whereas the environmentalists believe that the trade-offs have been set in the wrong direction and should be corrected towards increased protection. In addition, the major philosophy of the regulatory system is based on the conviction that regulators have the duty to broker a fair trade-off between economic benefits and environmental risk. As these three groups constitute the main prototypes described by the cultural theory of risk, it is unlikely that there is any institution or social force that would push hormesis into the policy arena (Douglas and Wildavsky 1982; Schwartz and Thompson 1990). If adopted by the scientific community, industrialists will hardly believe the "good" news, environmentalists will fight against the potential implications of more leniency towards polluters, and regulators will defend their previous approaches since institutions usually resist structural changes.

What does this situation mean for risk communication? What can the scientific community and public educators do if the hypothesis is further substantiated and validated? Obviously it is not sufficient to inform potential stakeholders or the public at large about the hormesis hypothesis and its implications. Most people will simply ignore the information or reject it. In my opinion two strategies might be helpful to get the message across:

• The hormesis hypothesis should first be introduced in connection with natural agents familiar to most people. It has been public knowledge that small doses of minerals and metals are necessary inputs for humans while larger quantities might be toxic. At the same time, people know about ingredients of medical drugs which are overall beneficial if taken in small doses. Once the message has been accepted with natural ingredients, the communication program may include substances that are regarded as natural but are normally synthesized in chemical processors. At the end, chemicals that are regarded as pollutants may enter the communication program once the principle has been accepted on the basis of natural ingredients.

• Any communication program should avoid to link the hormesis hypothesis with vested interests in the toxicological arena. Although the hypothesis may benefit industrial polluters by giving them a perfect excuse to deviate from any minimization concept, it is not clear whether the new regulatory regime of optimization does not require similar or even higher costs for industrial players. For example, regulators might demand a regional emissions profile in order to determine the overall exposure of individuals in a specific region. The combined exposure of all polluters should then be within the average range of beneficial effects. The means to implement such an optimal profile might be more cost-intensive than pursuing a conventional minimization strategy. If hormesis is being perceived as a new strategy of industry to avoid risk reduction measures and to gain points in court, the hypothesis will be rejected by most observers and there is little chance for a regulatory reform. Rather risk communication programs should stress the potential benefits of a regulatory regime that takes hormesis into account and make sure that the benefits are equally shared by industrialists, environmentalists, and the public affected by the regime. It needs to be proven that public health is served better if hormesis is applied to risk regulation and that the costs are equally shared among the interested parties.

Risk communication will not perform any miracles. It can help to overcome some of the perception biases that I outlined above and it has the potential to make people more susceptible to the benefits that go along with the hormesis theory. But it should be up to them and the legitimate policy bodies to decide on how to use this new information for policy making and regulation. The ideal target of risk communication is not the person who readily accepts and believes all the information given, but who processes all the available information to form a well-balanced judgment in accordance with the factual evidence, the arguments of all sides, and his/her own interests and preferences.


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