There are basically two types of Carbon Monoxide alarms commonly available on the market.  As with everything else the type providing superior protection are more expensive.

	Biomimetic type alarms consist of a gel coated disc that darkens in the presence of carbon monoxide.  A light beam through the gel senses a change in color and alarms.
	Electrochemical type alarms (also referred to as monitors) are much more accurate and may come with a digital readout.  More advanced models also have a memory feature that allows you to check past CO levels and the number of hours ago that level occurred.  This feature provides much more information as to the source of the CO.

Another important issue deals with the current UL listing.

Back in 1992, alarms were required to sound after 8 hours of exposure to 15 ppm's (at the lower end of the alarm requirements).

Unfortunately, many large metropolitan areas experience that level on a frequent basis and the first generation of alarms would sound due to outside, ambient levels.

As a result of these 'nuisance alarms', the most recent alarm standard (October 1, 1998 UL 2034) require that CO alarms sound when (at the low end) carbon monoxide levels remain at 30 ppm's for 30 days.

Many experts feel that this "one size fits all" UL standard may compromise protection for those who do not live in polluted areas of the planet.  

For more information on the past and current UL standards, click here.

Another important issue is what is an acceptable level for indoor, residential situations.  The EPA has set 9 ppm's as an ambient air quality goal averaged over 8 hours.

Unfortunately, many 'authorities or jurisdiction' have misinterpreted that to mean that anything below 9 ppm's is acceptable for inside a home.

Certainly, if carbon monoxide levels outside are, for example, 9 ppm's, the inside air would be expected to be similar.  However, if there is no measurable CO level outside, any increase in CO inside the home is cause for concern.

Any increase of CO levels inside the home, means two things:

	Carbon Monoxide is being produced, and
	There is a direct path for CO to reach the occupants of the dwelling.

This is why we feel very strongly that any increase between outside and inside levels of CO is a cause for immediate concern.

Also keep in mind that CO levels in a home may vary depending on a number of conditions.  For example, a test at 1 pm measures an eight ppm difference between outside and inside levels.  There is no assurance that, for example, at 2 o'clock in the morning the levels would have been measured at several hundred ppm's and that over time the CO dissipated due to outside doors being opened, etc. 

We recommend you purchase an alarm that provides the most protection for you and your family.  If you reside in a metropolitan area with substantial automotive and/or industrial influences, you may wish to consider an alarm which conforms to the 1998 UL 2034 Standard.

If you live in a more rural or suburban area, you may wish to consider an alarm/monitor which is more sensitive and has a digital display that reads down to under 10 ppm's although will not go into a full alarm mode until the 70 ppm level is attained for the prescribed amount of time.