Option 1: The option correctly infers that pollutants are a subset of harmful things. This means that all pollutants are harmful, but not all harmful things are pollutants. Therefore, this option is valid and can be inferred from the given statement.

Option 2: The option states that no pollutants are non-harmful. This aligns with the given statement, as it implies that all pollutants are harmful. Therefore, this option is also valid and can be inferred from the given statement.

Option 3: The option claims that if anything is harmful, it is a pollutant. This is not a valid inference, as it goes beyond the given statement. The given statement only states that all pollutants are harmful, but it does not imply that all harmful things are pollutants. Therefore, option 3 is invalid.

Option 4: The option asserts that some pollutants are harmful. This is a valid inference because the given statement states that all pollutants are harmful, so it implies that at least some pollutants are harmful. Therefore, option 4 is valid and can be inferred from the given statement.

In conclusion, the invalid inference is option 3. It is not valid to infer that anything harmful is a pollutant based on the given statement.