Using Commas

Use a comma

--to set off introductory words and phrases.
For reasons of safety, only company employees are allowed in the work area.
Whether the workers like it or not, they must wear hard hats in this area.
On Wednesday, please try to be at the site by 7:30 a.m.
--when a dependent clause comes first in a complex sentence.
When it is safe, air normally contains about 21% oxygen and 79% nitrogen.
After you have tested the air and found it safe, it is still prudent to have a breathing device with you.
--between the clauses linked by a coordinating conjunction (and, or, nor, but, so, for, while, yet) in a compound sentence (containing two or more independent clauses).
One worker moved the truck into the on-coming lane, and a second worker stopped the on-coming traffic.
(Without the comma, a serious accident might result.)
--to set off a nonrestrictive modifier.
Nonrestrictive: Vapor, which is the gaseous phase of a substance, can be injurious. Restrictive: Vapors or gases that are noxious are directly or indirectly harmful or destructive to the health or life of humans. Gases that are non-poisonous may asphyxiate simply by mechanically excluding oxygen.
--to set off explanatory material or definitions.
Workers must be outfitted with safety gear, hard hat and breathing equipment, before entering the sewer.
--to separate introductory words or phrases, where confusion might result.
As foreman, Petersen has been a great success.
--to set off an interjection or a transitional element.
Interjection: There are numerous hazards, unfortunately, that can cause physical injury in waste treatment plants.
Transition word: An atmosphere containing less than 19.5% oxygen by volume, however, is decidedly dangerous for humans
--to separate series of words, phrases, or clauses.
Natural gas, gasoline vapors, and hydrogen sulfide are all noxious hazards in the confined space of a sewer.
The places which are most likely to harbor dangerous noxious gases or oxygen deficiency are sewers with solid manhole covers, tightly-covered pits or chambers, and deep tanks or pump suction wells.
Note: The comma before the and in a series is optional. However, most technical writers use it consistently for clarity.
--to separate a series of coordinate adjectives.
Small, airless underground spaces are particularly susceptible to noxious gas buildup
Note: The comma here can be seen as taking the place of and. The words small and airless are called coordinate adjectives in that they both describe the noun phrase underground spaces. Therefore there is not comma to set off the word underground.

Do not use a comma

--to separate two independent clauses…creating a comma splice.
Wrong: The term sewer gas is a misnomer, it is not a single gas.
Correct: The term sewer gas is a misnomer since it is not a single gas.
Wrong: It is actually sewage sludge gas with a high content of carbon dioxide and varying amounts of methane, hydrogen, hydrogen sulfide and a small amount of oxygen, the hazard is usually from an explosive mixture of methane and oxygen or from an oxygen deficiency.
Correct: It is actually sewage sludge gas with a high content of carbon dioxide and varying amounts of methane, hydrogen, hydrogen sulfide and a small amount of oxygen. The hazard is usually from an explosive mixture of methane and oxygen or from an oxygen deficiency.
--when it separates a subject from its verb.
Wrong: Detection of an existing gas or vapor hazard and then taking steps to remove or to protect against it, are the sure ways to prevent accidents and loss of life.
Correct: Detection of an existing gas or vapor hazard and then taking steps to remove or to protect against it are the sure ways to prevent accidents and loss of life.

[Sources: Taken from my file, I wish I knew the source]