Perhaps you should try some spelling variations:
"50 ppm decrease" » "50 _ decrease" (Expand Search)
"5 ng decrease" » "5 _ decrease" (Expand Search), "5 g decrease" (Expand Search), "5 μg decrease" (Expand Search)
"_ a decrease" » "_ _ decrease" (Expand Search), "_ _ decreased" (Expand Search), "_ _ decreases" (Expand Search)
"50 c decrease" » "50 _ decrease" (Expand Search), "50 _ decreased" (Expand Search), "50 _ decreases" (Expand Search), "50 i 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 (((\"5 ng decrease\") OR (\"_ a decrease\"))))) OR (\"50 c decrease\"))~".
• Removing quotes may allow a broader search: (((( 50 ppm decrease ) OR ((( 5 ng decrease ) OR ( _ a decrease ))))) OR ( 50 c decrease ))~.
• Adding a wildcard symbol may retrieve word variants: (((("50 ppm decrease") OR ((("5 ng decrease") OR ("_ a decrease"))))) OR ("50 c decrease"))*.