Pollution is generated from the level of Waste produced on a planet. For every 200 Waste on a planet, your pollution level goes up.

Rulers, Specialists, Workers, Slaves, Robots, and Drones all produce 10 Waste.
Unemployed, Presapients, Biotrophies, and Percursors produce 5 Waste.
Pops produce 20 Waste while being purged and 10 while being assimilated.

Additionally, high pollution increases the appearance of environmental issues. They come in several forms:
Urban Pollution: These are the first to appear and impact how pleasant it is to live in the urbanized regions of the world.
Environmental Pollution: These start appearing as pollution increases and make the living conditions on the planet worse.
Natural Disasters: These appear more frequently with more pollution.
Climate Change: Heavily polluted worlds risk collapsing their planet's natural environment entirely.

Pollution can be dealt with by reducing the waste on a planet. There are various tools to do this.

Waste Processing Buildings
Waste recovery buildings can be built to employ waste workers, who lower the Waste on a planet by 20 as well as provide small amounts of recycled materials. The building can be upgraded several times. The final building requires dark matter to power.

Junkyards
These buildings can be built as much as you'd like on your worlds, further increases the benefits of your waste workers.

Environmental Policy
You can set your environmental policy to focus more on production or on waste prevention. Some ethics get stronger environmental policies, while other ethics get stronger production policies.

Recycling Policy
You can change what your waste workers do with the waste they process. This replaces the Recycling Campaign. You can focus on energy, material recycling, product, or food recycling; or just dispose of it all.

National Parks
You can set aside a district to act as a natural buffer against pollution. It provides ranger jobs and amenities.

Governors
The Ecologist can reduce the job upkeep on a planet and lower your demand on production for manufacturing and research.

Harmful Environments
You can remove blockers that may be harmful to the natural environment, such as industrial wastelands, floating garbage heaps, or abandoned strip mines.

Technology
You can increase the efficiency that waste workers convert waste into usable materials through research.

Garbage Worlds
You can send waste to a single location for processing. The pollution levels of the garbage world will be very high, so it will need lots of waste management.

Ascension Perk
You can adopt the Land Ethic perk and establish wildlife preserves and get a head start on conservation research.

Civics
The Environmentalist civic allows you to gain Unity with Waste Workers and Rangers. Hive Minds can be Scavengers to turn Waste into food, machine empires can be Recyclers and improve their production of recycled materials, and megacorps can be Scrappers and sell their Waste.

Traits
Biological and mechanical traits for making your species produce more or less Waste.

Planets can be affected by a large number of different environmental conditions and have varying environmental strengths and vulnerabilities.

Ecological Resistance
Worlds with resistant environments are less affected by pollution, while fragile ecosystems react more often to disturbances. A planet's ecological resilience can increase or decrease depending on its pollution levels.

Terraformed Worlds
Terraforming replaces the previous ecosystem with an entirely new one. The results are unpredictable.

Terraforming Candidates
Barren worlds occasionally have the terraforming candidate modifier from the start of the game.

Geoengineering
Many undesirable modifiers can be removed with various geoenginering projects. These are unlocked through the various terraforming techs and add decisions to worlds with modifiers that can be removed.

Environmental Ethic
During the late game you'll have the option to research an environmental ethic based on the ethic or authority of your empire. Once completed, you'll have a unique mechanic for dealing with pollution based on the kind of empire you have.

You can now get additional resources from the natural environment if you don't care about dealing with the consequences.

Rich Ecosystems
Planets have a chance to generate with new blockers that can be removed to gain strategic resources, but removing those blockers negatively impacts the environment.

Urban Sprawl
The first time a world reaches its maximum district size, you have the option to continue expanding into its natural environment and keep adding districts at an environmental cost. Can be removed with civil engineering.

Overconsumption
Planetary decision that replaces Encourage Planetary Growth. Increases population upkeep for amenities and population growth.

Slums
Overpoulated worlds will create slums for the homeless and criminal jobs to prey on them. These slums can actually increase the housing and employment of your worlds if you don't mind the crime and pollution.