Select the Advanced Search in each of the databases, and combine terms such as "lead poisoning" AND children or "lady's slipper" AND germination. Put words in quotation marks to search phrases.
Hints: If an obvious keyword (for example, "microplastic*") does not work, try searching variations of the word (such as, "plastic"). Combine with other terms, and use synonyms or more general terms (something like, "mussels" or "marine").
Image credit: The Story of Mathematics (Luke Mastin): http://www.storyofmathematics.com/19th_boole.html