Learn Finnish: Kyltti varoittaa vaaratekijästä, altistumisesta ja tapaturmasta. - The sign warns about a hazard, exposure, and an accident.

This is because of the verb varoittaa (to warn). Verbs of warning in Finnish require the thing you are being warned about to be in the elative case. This is a fixed verb rection you just have to memorize.

Do we have to add the elative ending to every single noun in the list?

Yes. Unlike English where you can say 'warns about A, B, and C' using 'about' just once, Finnish grammar requires every noun in the list to take the correct case ending. That is why you see vaaratekijästä, altistumisesta, and tapaturmasta.

How does the word altistuminen become altistumisesta?

Altistuminen (exposure) is a noun ending in -nen. To add a case ending, the -nen changes to an -se- stem. So, altistuminen becomes altistumise- plus the elative ending -sta.

Why does vaaratekijästä use the -stä ending instead of -sta?

This is due to vowel harmony. The word vaaratekijä is a compound word (vaara + tekijä). In compound words, vowel harmony is determined by the final word in the compound. Since tekijä contains front vowels (e, i, ä), it takes the front-vowel ending -stä.