Perhaps you should try some spelling variations:
"20 means decrease" » "20 mean decrease" (Expand Search), "20 deaths decreased" (Expand Search), "_ means decrease" (Expand Search), "k means decrease" (Expand Search), "i means decrease" (Expand Search)
"a means decrease" » "a mean decrease" (Expand Search), "a deaths decreased" (Expand Search), "_ means decrease" (Expand Search), "k means decrease" (Expand Search)
"50 ppm decrease" » "50 _ decrease" (Expand Search), "50 a decrease" (Expand Search), "50 nn decrease" (Expand Search), "5 ppm decrease" (Expand Search)

You may be able to get more results by adjusting your search query.

• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"50 ppm decrease\") OR (((\"20 means decrease\") OR (\"a means decrease\"))))~".
• Removing quotes may allow a broader search: (( 50 ppm decrease ) OR ((( 20 means decrease ) OR ( a means decrease ))))~.
• Adding a wildcard symbol may retrieve word variants: (("50 ppm decrease") OR ((("20 means decrease") OR ("a means decrease"))))*.