Each of these sources has its own strengths and weaknesses.
Newspapers: Written for general readers on a wide variety of topics.
Example: Newspaper article reporting local pollution event, including interviews with residents.
Magazines: Also for current topics, written for general readers with no background in the subject. Example: Longer, in depth article about a pollutant or pollution event for a general audience.
Academic Journals: Longer research reports by specialists, written for other specialists. Can be quite technical. (Also called Scholarly / Peer Reviewed / Professional Journals). Example: Research article examining the detrimental health effects of overexposure to lead.
Books: Much longer and in-depth than articles from newspapers, journals and magazines.Can be written for the non-specialist or specialist. Due to publishing time, will probably not cover recent events. Some may be collections of individual articles. Example: Book on importance of access to clean drinking water.
Regulations: Regulations, legislation, rules, standards and advisories from health or other government organizations will give you an idea of what are currently considered acceptable levels of exposure, limits on discharge, handling and disposal requirements, projected goals and other regulatory policies. Example: Recommended thresholds for exposure to lead by the Centers for Disease Control and Prevention.
Statistics: Data of how much, how many, how often provided in clear charts. Often produced by government organizations. When you find a number quoted in an article, see if you can find the original source. Example: U.S. greenhouse emissions by gas (1990-2105) from the EPA.