Pick the important keywords or phrases to describe your topic.
Motor vehicles are major sources of gas-phase organic carbon, which includes volatile organic compounds. These emissions react in the atmosphere, leading to the formation of ozone.
Consider alternate terms.
motor vehicles or automobiles
high blood pressure or hypertension
Some databases search only the words you enter so you may miss other relevant papers. Many will allow you to enter a wild card character to search variations on the term.
robot* = robot, robots, robotics
Many databases will locate the keywords you enter anywhere in the citation. Consider focusing your results by entering an exact phrase using quotation marks.