This is the 15743rd most frequent Polish word.
szkodliwymi
"Harmful" or "damaging" (instrumental plural form).
The word 'szkodliwymi' describes the characteristic of the substances being harmful, in instrumental plural form, emphasizing their negative impact.
Te chemikalie są bardzo szkodliwymi substancjami dla środowiska.
These chemicals are very harmful substances for the environment.
The word 'szkodliwymi' used here highlights the negative nature of the materials, again in instrumental plural form due to the grammatical requirements.
Musimy unikać kontaktu z szkodliwymi materiałami.
We must avoid contact with harmful materials.
In this case, 'szkodliwymi' is instrumental plural, describing products identified with the harmful attribute or characteristic.