Pollution in the free game C-evo has (unlike its equivalent in some commercial games) almost no random element. (The on-screen manual has no separate page about pollution. A search for "pollu" or "pollution" displays a list of ten pages on which it is mentioned.)
There is no pollution under Despotism (nor, presumably, under Anarchy because no tiles are being exploited).
The basic statement from the on-screen manual is "Each exploited material resource causes one pollution point". You subtract from that the number of citizens (each of whom can absorb one point). Surplus pollution builds up and is shown in a bar near the left-hand side of the city screen.
When 240 or more points have built up (as indicated by the star the previous turn), 240 points are subtracted in exchange for the pollution of one land tile in the city radius, generally one of those with the greatest material output (whether or not it is currently exploited). That tile displays the "polluted" symbol and loses half of its material and food output. (The food part can be critical for a city that has little spare food. The manual seems not to warn specifically about that, but it uses the possibly ambiguous wording "losing half of its resource output".)
A captured city retains its pollution total.
Buildings that increase or decrease pollution
A Factory, a Manufacturing Plant, and two of the power plants affect pollution. (The manual does not say whether the percentages are before or after the population deduction.)
A Factory or Power Station increases pollution by 50%. A Hydroelectric Dam reduces it by 50%. A Nuclear Plant (C-evo) costs less than those to maintain but does not affect pollution. The Hoover Dam counts as a Hydroelectric Dam in every city.
Clean-up and prevention
An engineer or settler unit can clean up pollution (with the "P" key), taking 18 movement points.
Prevention of pollution is possible only with a Recycling center (C-evo).