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The term "non-excludable" is a key characteristic of public goods. It describes any resource or service that cannot be limited to specific individuals; everyone has equal access. This means that no one can be effectively stopped or "excluded" from using it.
For instance, when it comes to national defense, every citizen in the country benefits from the nation's protective forces. This benefit is shared regardless of personal contribution to defense budgets or taxes, making the service non-excludable.
Similarly, clean air is available to all. You cannot restrict access to air for any particular individual without providing it to others in an area. Hence, clean air remains accessible to everyone at all times.

The concept of "non-rivalrous" applies when one individual's usage of a good doesn't diminish its availability for others. This is another defining feature of public goods.
In the case of national defense, the fact that one person or group feels safe does not reduce the protection felt by others. The security a country enjoys is shared simultaneously among all citizens without contest or depletion.
Similarly, for clean air, one person's breathing doesn't take away the air available for another person. The quality and quantity of the air remain largely unaffected by individual consumption, ensuring that it remains non-rivalrous.

National defense serves as a prime example of a public good because it embodies both non-excludability and non-rivalry. Every individual within a nation's borders benefits from its protection mechanisms, such as military defense, regardless of whether they personally contributed to its funding.
The benefits derived from national defense protect the entire community, maintaining the safety and sovereignty of the nation. Because no one is excluded from this protection and it does not diminish when more people avail it, national defense is categorized as a public good.

• Provides public security
• Involves protective services like military and intelligence
• Benefits are universally distributed

Clean air is universally considered a public good due to its inherent qualities of being non-excludable and non-rivalrous. The air we breathe circulates freely and is accessible to everyone, illustrating the non-excludability aspect. Attempts to restrict air use would be impractical and unattractive.
Moreover, breathing clean air doesn't diminish its availability for others, upholding its non-rivalrous nature. This mutual availability ensures that all individuals within a community enjoy the same quality of air, contributing to the overall health and well-being of the environment.

• Provides vital life support
• Essential for public health
• Contributes to environmental sustainability