Precautionary principle
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The precautionary principle is a moral and political principle which states that if an action or policy might cause severe or irreversible harm to the public, in the absence of a scientific consensus that harm would not ensue, the burden of proof falls on those who would advocate taking the action. It aims to provide guidance for protecting public health and the environment in the face of uncertain risks, stating that the absence of full scientific certainty shall not be used as a reason to postpone measures where there is a risk of serious or irreversible harm to public health or the environment.
An alternate formulation states that the lack of certainty regarding the threat should not be used as an excuse to do nothing to avert that threat. Says P. Saunders, all this principle actually amounts to is "if one is embarking on something new, one should think very carefully about whether it is safe or not, and should not go ahead until reasonably convinced it is."

# Other formulations of the precautionary principle
The 1998 Wingspread Statement on the Precautionary Principle summarizes the principle this way: "When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically." (The Wingspread Conference on the Precautionary Principle was convened by the Science and Environmental Health Network).
The February 2, 2000 European Commission Communication on the Precautionary Principle notes: "The precautionary principle applies where scientific evidence is insufficient, inconclusive or uncertain and preliminary scientific evaluation indicates that there are reasonable grounds for concern that the potentially dangerous effects on the environment, human, animal or plant health may be inconsistent with the high level of protection chosen by the EU".
The January 29, 2000 Cartagena Protocol on Biosafety says: "Lack of scientific certainty due to insufficient relevant scientific information . . . shall not prevent the Party of import, in order to avoid or minimize such potential adverse effects, from taking a decision, as appropriate, with regard to the import of the living modified organism in question."
It is important to emphasize that although this principle operates in the context of scientific uncertainty, it is considered by its proponents to be applicable only when, on the basis of the best scientific advice available, there is good reason to believe that harmful effects might occur.
The precautionary principle is most often applied in the context of the impact of human actions on the environment and human health, as both involve complex systems where the consequences of actions may be unpredictable.
As applied to environmental policy, the precautionary principle stipulates that for practices such as the release of radiation or toxins or massive deforestation the burden of proof lies with the advocates.   Concerning potential risks to public health, examples of cases in which the precautionary principle has been advocated (but not always accepted) are: the commercialization of genetically modified foods, the use of growth hormones in cattle raising, measures to prevent the "mad cow" disease, health claims linked to phthalates in PVC toys, among many others.
An important element of the precautionary principle is that its most meaningful applications pertain to those that are potentially irreversible, for example where biodiversity may be reduced.  With respect to bans on substances like mercury in thermometers, freon in refrigeration, or even carbon dioxide exhaust from automobile engines and power plants, it implies:
... a willingness to take action in advance of scientific proof  evidence of the need for the proposed action on the grounds that further delay will prove ultimately most costly to society and nature, and, in the longer term, selfish and unfair to future generations.
The concept includes ethical responsibilities towards maintaining the integrity of natural systems, and the fallibility of human understanding.
Some environmental commentators take a more stringent interpretation of the precautionary principle, stating that proponents of a new potentially harmful technology must show the new technology is without major harm before the new technology is used.

# Origins and theory
The formal concept evolved out of the German socio-legal tradition in the 1930s, centering on the concept of good household management. In German the concept is Vorsorgeprinzip, which translates into English as precaution principle.
Many of the concepts underpinning the precautionary principle pre-date the term's inception. For example, the essence of the principle is captured in a number of cautionary aphorisms such as "an ounce of prevention is worth a pound of cure", "better safe than sorry", and "look before you leap". The precautionary principle may also be interpreted as the evolution of the ancient medical principle of "first, do no harm" to apply to institutions and institutional decision-making processes rather than individuals.
In economics, the precautionary principle has been analysed in terms of the effect on rational decision-making of the interaction of irreversibility and uncertainty. Authors such as Epstein (1980) and Arrow and Fischer (1974) show that irreversibility of possible future consequences creates a quasi-option effect which should induce a "risk-neutral" society to favor current decisions that allow for more flexibility in the future. Gollier et al (2000) conclude that "more scientific uncertainty as to the distribution of a future risk  that is, a larger variability of beliefs  should induce Society to take stronger prevention measures today."

# Application
The application of the precautionary principle is hampered by the wide range of interpretations placed on it. One study identified 14 different formulations of the principle in treaties and nontreaty declarations. R.B. Stewart (2002) reduced the precautionary principle to four basic versions:
- Scientific uncertainty should not automatically preclude regulation of activities that pose a potential risk of significant harm (Non-Preclusion PP).
- Regulatory controls should incorporate a margin of safety; activities should be limited below the level at which no adverse effect has been observed or predicted (Margin of Safety PP).
- Activities that present an uncertain potential for significant harm should be subject to best technology available requirements to minimize the risk of harm unless the proponent of the activity shows that they present no appreciable risk of harm (BAT PP).
- Activities that present an uncertain potential for significant harm should be prohibited unless the proponent of the activity shows that it presents no appreciable risk of harm (Prohibitory PP).
In deciding how to apply the principle, analysis may use a cost-benefit analysis that factors in both the opportunity cost of not acting, and the option value of waiting for further information before acting. One of the difficulties of the application of the principle in modern policy-making is that there is often an irreducible conflict between different interests, so that the debate necessarily involves politics.

## International agreements and declarations
The World Charter for Nature, which was adopted by the UN General Assembly in 1982, was the first international endorsement of the precautionary principle. The principle was implemented in an international treaty as early as the 1987 Montreal Protocol, and among other international treaties and declarations  is reflected in the 1992 Rio Declaration on Environment and Development (signed at the United Nations Conference on Environment and Development).
European Commission
On 2 February 2000, the European Commission issued a Communication on the precautionary principle, in which it adopted a procedure for the application this concept, but without giving a detailed definition of it.  Earlier, the Maastricht Treaty adopted the principle as a fundamental element of environmental policy: Article III-233 of the draft Treaty establishing a constitution for Europe :
After the adoption of the European Commission's Communication on the precautionary principle, the principle has come to inform much EU policy, including that in areas beyond that of environmental policy. It is implemented, for example, in the EU food law and also affects, among others, policies relating to consumer protection, trade and research, and technological development. While a comprehensive definition of the precautionary principle was never formally adopted by the EU, a working definition and implementation strategy for the EU context has been proposed by Rene von Schomberg in Fisher et al. (2006):
"Where, following an assessment of available scientific information, there are reasonable grounds for concern for the possibility of adverse effects but scientific uncertainty persists, provisional risk management measures based on a broad cost/benefit analysis whereby priority will be given to human health and the environment, necessary to ensure the chosen high level of protection in the Community and proportionate to this level of protection, may be adopted, pending further scientific information for a more comprehensive risk assessment, without having to wait until the reality and seriousness of those adverse effects become fully apparent".
USA
On July, 18, 2005, the City of San Francisco passed a Precautionary Principle Purchasing ordinance, which requires the city to weigh the environmental and health costs of its $600 million in annual purchases  for everything from cleaning supplies to computers. Members of the Bay Area Working Group on the Precautionary Principle including the Breast Cancer Fund, helped bring this to fruition.
Corporate
The Body Shop International, a UK-based cosmetics company, recently included the Precautionary Principle in their 2006 Chemicals Strategy.

## Environment/health
Fields typically concerned by the precautionary principle are the possibility of:
- Global warming or other abrupt climate change
- Extinction of species
- Introduction of new and potentially harmful products into the environment, threatening biodiversity (e.g., genetically modified organisms)
- Threats to public health, due to new diseases and techniques (e.g., AIDS transmitted through blood transfusion)
- Persistent or acute pollution (asbestos, endocrine disruptors...)
- Food safety (e.g., Creutzfeldt-Jakob disease)
- Other new biosafety issues (e.g., artificial life, new molecules)
- Threat of an uncontrolled insane future superintelligence
The precautionary principle is often applied to biological fields because changes cannot be easily contained and have the potential of being global. The principle has less relevance to contained fields such as aeronautics, where the few people undergoing risk have given informed consent (e.g., a test pilot). In the case of technological innovation, containment of impact tends to be more difficult if that technology can self-replicate. Bill Joy emphasized the dangers of replicating genetic technology, nanotechnology, and robotic technology in his article in Wired Magazine, "Why the future doesn't need us", though he does not specifically cite the precautionary principle.  The application of the principle can be seen in the public policy of requiring pharmaceutical companies to carry out clinical trials to show that new medications are safe.
Oxford based philosopher Nick Bostrom discusses the idea of a future powerful superintelligence, and the risks that we/it face should it attempt to gain atomic level control of matter..
Application of the principle modifies the status of innovation and risk assessment: it is not the risk that must be avoided or amended, but a potential risk that must be prevented. Thus, in the case of regulation of scientific research, there is a third party beyond the scientist and the regulator: the consumer.
In an analysis concerning application of the precautionary principle to nanotechnology, Chris Phoenix and Mike Treder posit that there are two forms of the principle, which they call the "strict form" and the "active form". The former "requires inaction when action might pose a risk", while the latter means "choosing less risky alternatives when they are available, and  taking responsibility for potential risks."

## Change of laws controlling societal norms
Associate Justice Martha Sosman's dissent  in Goodridge v. Department of Public Health, the decision of the Supreme Judicial Court of Massachusetts that mandated legalization of same sex marriage, is an example of the precautionary principle as applied by analogy to changes in culturally significant social policy. She describes the myriad societal structures that rest on the institution of marriage, and points out the uncertainty of how they will be affected by this re-definition. The disagreement of the majority illustrates the difficulty of reaching agreement on the value of competing perspectives. Although the Goodridge case involved interpreting the state constitution, the substantive canon in Anglo-American jurisprudence that derogations of fundamental societal values should be narrowly construed  is analogous to the precautionary principle favoring a statutory interpretation that comports with rather than damages the common law and established norms. See, for example, Holy Trinity Church v. United States, 143 U.S. 457 (1892).

## Resource management
Several natural resources like fish stocks are now managed by precautionary approach, through Harvest Control Rules (HCR) based upon the precautionary principle. The figure indicates how the principle is implemented in the cod fisheries management proposed by the International Council for the Exploration of the Sea.
In classifying endangered species, the precautionary principle means that if there is doubt about an animal's or plant's exact conservation status, the one that would cause the strongest protective measures to be realized should be chosen. Thus, a species like the Silvery Pigeon that might exist in considerable numbers and simply be under-recorded or might just as probably be long extinct is not classified as "data deficient" or "extinct" (which both do not require any protective action to be taken), but as "critically endangered" (the conservation status that confers the need for the strongest protection), whereas the increasingly rare, but probably not yet endangered Emerald Starling is classified as "data deficient", because there is urgent need for research to clarify its status rather than for conservation action to save it from extinction.

# Criticisms

## Threshold of plausibility
The Wingspread Statement version of the PP takes the form "When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically". Does mankind need a minimal threshold of scientific certainty or plausibility before undertaking preventative action?  Normally, no minimal threshold of plausibility is specified as a triggering condition, so that even the slightest indication that a particular product or activity might possibly produce some harm to human health or the environment will suffice to invoke the principle. And just as often no other preventative action is contemplated than an outright ban on the incriminated product or activity .

## Negative consequences of poor application
In some applications, the Precautionary Principle may cause more harm than it alleviates. This is because people are more acutely aware of negative changes than they are positive changes.  Because of this effect, a technology which brings great advantages may be ruled out by the Precautionary Principle because of its potential for severe negative impacts, leaving the overriding positive benefits unrealized.
The Hazardous Air Pollutant provisions in the 1990 amendments to the U.S. Clean Air Act are an example of the Precautionary Principle where the onus is now on showing a listed compound is harmless. Under this rule no distinction is made between those Hazardous Air Pollutants that provide a higher or lower risk and there is a perverse incentive to use less well studied agents that are not on the existing list.

## Perspective
- Critics of the principle argue that it is impractical, since every implementation of a technology carries some risk of negative consequences. 
Proponents counter that the principle is not an absolute rule, it is a conceptual tool to clarify arguments, and especially an issue of where the burden of proof lies.
- Proponents counter that the principle is not an absolute rule, it is a conceptual tool to clarify arguments, and especially an issue of where the burden of proof lies.
- Critics also claim that proponents are sometimes selective in their use of the precautionary principle  applying it for political reasons, rather than scientific reasons. They note that the selective application of principles in government is considered a fundamental form of injustice, which is why selective enforcement is considered an abuse of power.
Proponents counter that acting with insufficient knowledge of risk (e.g., spreading new lifeforms into the biosphere) can also be socially unfair, costly, and detrimental when applied to complex environmental choices. They note that the reckless application of technology is a worse abuse of power.
- Proponents counter that acting with insufficient knowledge of risk (e.g., spreading new lifeforms into the biosphere) can also be socially unfair, costly, and detrimental when applied to complex environmental choices. They note that the reckless application of technology is a worse abuse of power.
- A well-cited summary of some representative objections to the precautionary principle are described in a Reason article by Ronald Bailey which, using the Wingspread consensus as a starting point, argues the possibilities for misapplication of the principle.