Human subjects research is any research that systematically collects data or information about living individuals through intervention, interaction, or the use of identifiable private information with the intent of developing or adding to generalizable knowledge. 

EPA Regulation, codified at 40 CFR Part 26, defines the terms research and human subject as follows: 

• Research is a systematic investigation, including research development, testing and evaluation, designed to develop or contribute to generalizable knowledge (40 CFR 26.102.d).

• Human subject means a living individual about whom an investigator (whether professional or student) conducting research:  (i) Obtains information or biospecimens through intervention or interaction with the individual, and uses, studies, or analyzes the information or biospecimens; or (ii) Obtains, uses, studies, analyzes, or generates identifiable private information or identifiable biospecimens. [40 CFR 26.102(e)]

It is very important to make the human subjects research determination by proceeding in a particular order. This is because if your project does not constitute research, then it does not matter if the project involves human subjects! Only if a project is both considered research AND involves human subjects (both according to the federal definitions) is the project then considered human subjects research and subject to additional oversight.