The importance of clean breathing air to sustain life and maintain good health is well known.  The dangers associated with breathing contaminated air are also well known and especially critical when using SCBA tanks in emergency situations.  Some of the common Air Contamination Sources are listed below.

Sigma Sensing has individual solutions for each contaminant or solutions for monitoring multiple contaminants.

Normal CO2 levels outdoors (i.e. 200 - 400 ppm) or indoors (i.e. 500 - 2,500 ppm) are not considered hazardous. However, compressed air with CO2 levels that are within the "indoors range" can create problems in Self-Contained Breathing Apparatus (SCBA) applications. Some compressors might be equipped with filters to reduce CO2 levels. CGA G-7.1 lists a 1,000 ppm maximum for Grade D and a 500 ppm maximum for Grade E air. High CO2 levels in SCBA tanks can produce many of the same symptoms as CO poisoning. In addition, high CO2 levels increase breathing rates, which shorten SCBA usage time. One of the most common causes of SCBA air quality failures is excessive CO2 content.

Carbon dioxide stimulates the respiratory center.  A buildup of CO2 in breathing air increases the breathing rate, which can deplete SCBA air supply more rapidly and increase inhalation of other contaminants.
The CO2 concentration is the more crucial than the lack of O2 for controlling respiration because sensors in the carotid artery (which supplies blood to the brain) detect changes in the partial pressure of both O2 and CO2 (PO2 and PCO2).  However, PO2 must be reduced by about half before the carotid artery sensors send a message to the respiratory control center in the medulla to breathe harder.  
The cerebrospinal fluid, which surrounds the brain and spinal column is much more sensitive to changes in respiration than sensors in the carotid artery.  It monitors PCO2 levels in the blood indirectly by monitoring hydrogen ion concentrations.  Most CO2 in the blood is in the ionized form as carbonic acid and a hydrogen ion (CO3─plus H+ ).  The cerebrospinal fluid sensors actually monitor the hydrogen ion concentration.  When hydrogen ion concentration changes, these sensors send messages to the respiratory control center almost instantaneously, which immediately restore proper respiration.  Therefore, the CO2 concentration is more crucial than the lack of O2 for controlling respiration.
High CO2 levels can be indicative of compressor problems.  Carbon monoxide is converted to CO2 by hopcalite in the compressor CO filter.  Therefore, high concentrations of CO2 can result from the hopcalite catalyzing elevated concentrations of CO. 
Recommended maximum Carbon dioxide (CO2) 1000 ppm

Carbon monoxide (CO) which is termed a toxic asphyxiant, is a colorless, odorless gas that is  invisible to the human senses and ranks as the most dangerous compressed air contaminant. Low ppm doses of carbon monoxide can cause headaches and dizziness High doses can be fatal. High-pressure compressors are often equipped with a catalyst which converts CO into much less toxic CO2.

Because CO is colorless and odorless, it is impossible for respirator wearers to detect.  CO combines readily with hemoglobin in red blood cells and prevents the transfer of oxygen to the tissues, causing oxygen starvation or hypoxia.   Possible sources of CO include:
Motor exhaust drawn into compressor air intake; Generated within compressors as combustion product of fuels, lubricants and overheated oils;
Generated within compressors from oxidation of overheated sorbent filters.  CO accumulated on a filter can be released when there is a drop in operating pressure
Both CGA G-7.1 Grades D and E (the most widely recognized SCBA air quality grades) list a 10 ppm maximum CO content.  European Pharmacopoeia lists < 5 ppm (v/v).

The dew point is the temperature at which water vapor will start to condense from air. This value depends upon the air's water vapor content and pressure. The water vapor content of intake air ranges from saturated to very dry.
Basically, there should be no liquid water in the breathing air to prevent freezing in atmosphere-supplying respirators.
The lower the dew point, the lower the moisture content.

Oil was formerly called condensed hydrocarbons
Large particles of condensed hydrocarbons, or oil, can be removed by the body's clearance mechanisms (i.e., phagocytosis and mucociliary escalator).  Smaller oil particles are retained, and may be hazardous, depending on the type and amount.  Oil mist deposits in the alveoli can cause an intense inflammation, known as lipoidpneumonia.   Oil mist can also cause emphysema by dilating and rupturing the alveoli, thus decreasing the total surface area available for the transfer of oxygen and carbon dioxide.

Oil is a major contaminant in systems using lubricated compressors. In reciprocating compressors, lubricating oil applied to cylinders causes small droplets by the shearing action of the piston to enter the air system as a mist. Oil mist can cause breathing discomfort, nausea and pneumonia, and create unpleasant taste and odors. Centrifugal compressors are “oil-free compressors”, but the term  refers only to the compression chamber, not the compressor system as a whole, or the resulting compressed air quality.

Possible oil sources include dust and pollen, motor exhaust pulled into the compressor air intake, and oil generated inside the compressor if lubricants escape through faulty piston rings.

Breathing apparatus (EN 12021): 0.5 mg/m³