Liability and state responsibility rules determine whether the 'polluter pays principle' has affected the making of international law. The relevant question here is: does the polluter really pay when an accident happens, the consequences of which affect human health or the environment? From the instruments that have been adopted one gets a mixed picture. Most international regimes channel liability to the person in control of an environmentally damaging activity. In the case of oil pollution and the sea transfer of dangerous substances, the person in control is the shipowner. In the case of nuclear pollution, the person in control is the operator of the nuclear power plant. In the case of the carriage of dangerous goods, it is carrier of dangerous goods. In the case of waste exports, the person who gives notification to the country of destination that a waste transfer is to take place is the person who is liable until the disposer takes control of the waste. From that point on, the disposer is considered the liable person.
While holding the person in control of a dangerous activity liable, most regimes also apply a broader version of polluter pays principle that views society at large that benefits from industrialization as accountable. Most regimes provide additional compensatory mechanisms in the form of funds financed by states or entities that, while they are not directly in control of a polluting activity, could be considered to benefit from it. In the oil pollution regime, for instance, a fund is created through which oil receiving industries contribute money to supplement compensation provided by the shipowner. In the case of nuclear power the states in which nuclear power plants are located have gotten together to establish funds that provide compensation in case of a nuclear accident.
Further issues have to do with state responsibility for acts that a state has undertaken that are considered wrongful under international law.
Policymakers have further engaged in efforts to establish whether a state could be held primarily liable for ultra-hazardous activities taking place in its territory under a regime of "international liability."
International law is notoriously weak on enforcement. It is often said that international law cannot be enforced since there is no international police force to enforce decisions of international tribunals and to punish states for the transgression of rules of international law. In international law, the makers of the law, that is states, are those that have to enforce the law. States are usually reluctant to pursue enforcement action against other states unless the transgression of international rules affects their vital interests. Furthermore, the purpose of international law is the preservation of peace. If states started to enforce international law to the point of executing acts of aggression against other states, international law would start to look incoherent.
International law is an incentive-based instrument. States often follow international law because doing otherwise would affect their reputation. States also follow international law because they know that their failure to follow the rules will be an invitation for other states to demonstrate non-compliance, something that is not in their interests. States tend to demonstrate high vigilance for the monitoring and enforcement of international treaties on the proliferation of weapons of mass destruction but rarely do they demonstrate the same vigilance for the enforcement of environmental treaties. In a world in which regional conflicts, terrorism, and various diseases are still prevalent the protection of environment does not always get the priority it deserves.
Having said that, states are increasingly undertaking obligations that could make a difference in the protection of environment. Many international treaties contain provisions that provide that states have to notify and consult with other states for activities that they undertake in their territory that are likely to have adverse consequences for other states. States are to execute Environmental Impact Assessments and Strategic Impact Assessments for plans and projects likely to have adverse effects on the environment of other states.
Certain international treaties provide a mixture of sticks and carrots to ensure state compliance. In the ozone regime, for instance, a multilateral fund is established that provides assistance to developing countries to comply with the regime. However, if the enticement provided by the fund does not suffice states are entitled to impose trade sanctions on states that have not complied. A number of other instruments are increasingly adopting compliance procedures the purpose of which is to identify the causes of noncompliance and respond accordingly either with financial assistance or with sanctions.