Lists, carcinogens
When we consider human health effects, one of the most worrisome is cancer. Cancer is very common in industrial societies, about one-third of citizens of wealthy countries will die of cancer. There are certain substances that, in high, repeated doses are known to cause cancer. Tobacco smoke is one such substance, asbestos is another. Benzene, a component of gasoline, also is known to cause cancer. All cancer causing substances would be considered hazardous.
Here is a list substances that are known and suspected to cause cancer https://ntp.niehs.nih.gov/pubhealth/roc/index-1.html f You can get more information, including how those lists were made and what "anticipated" means from Introduction, Page 5. You'll note that there are less than 100 substances listed as known. Most of them are "chemicals." Another list of carcinogens is available from IARC, International Agency for Research on Cancer, which is a World Health Organization (United Nations), agency. IARC has similar lists of substances, in Group 1 there is a list of Exposure Situations (i.e., jobs) that cause cancer. Look at those.
Here is another list of carcinogens. It is composed by the State of California, pursuant to a California law called Proposition 65. It lists over 500 substances that the State of California believes cause cancer. (The links are on the bottom of the page.) Also listed are about 500 substances that are believed to cause birth defects or reproductive disorders. The links to the actual lists are at the bottom of the page.
There are many similar lists, and we will use some of them as needed. Also we will learn some ways to find out if substances, generally chemicals, are on such lists. Important point here is that, as a practical matter, we usually determine hazards and the level of risk by looking up chemicals in the appropriate list, rather than de novo experiments, calculations, or literature searching. Because a chemical is on one of these lists does not mean there is evidence it causes cancer or any other dreaded problem in humans under normal exposure circumstances - remember Paracelsus.
How many chemicals are there? CAS has data on 33 million different chemicals. About 3 million chemicals are available and about 30,000 are "common," i.e., used in commerce. How many chemicals are known to be harmful or benign? The gruesome truth is that less than 1,000 chemicals have adequate toxicity testing. A few thousand more have some testing. Knowledge about the rest is based on guesswork perhaps relating its structure or other properties to data on chemicals with similar structure or properties, about which something is known.