Particle pollution (also known as "particulate matter") consists of a mixture of solids and liquid droplets. Some particles are emitted directly; others form when pollutants emitted by various sources react in the atmosphere. Particle pollution levels can be very unhealthy and even hazardous during events such as forest fires. Particle levels can be elevated indoors, especially when outdoor particle levels are high.

Particles come in a wide range of sizes. Those less than 10 micrometers in diameter (smaller than the width of a single human hair) are so small that they can get into the lungs, where they can cause serious health problems.

The smallest particles (those 2.5 micrometers or less in diameter) are called "fine" particles. These particles are so small they can be detected only with an electron microscope. Major sources of fine particles include motor vehicles, power plants, residential wood burning, forest fires, agricultural burning, some industrial processes, and other combustion processes.

Particles between 2.5 and 10 micrometers in diameter are referred to as "coarse." Sources of coarse particles include crushing or grinding operations, and dust stirred up by vehicles traveling on roads.

Particles smaller than 10 micrometers in diameter can cause or aggravate a number of health problems and have been linked with illnesses and deaths from heart or lung disease. These effects have been associated with both short-term exposures (usually over 24 hours, but possibly as short as one hour) and long-term exposures (years).

Sensitive groups for particle pollution include people with heart or lung disease (including heart failure and coronary artery disease, or asthma and chronic obstructive pulmonary disease), older adults (who may have undiagnosed heart or lung disease), and children. The risk of heart attacks, and thus the risk from particle pollution, may begin as early as the mid-40s for men and mid-50s for women.

When exposed to particle pollution, people with heart or lung diseases and older adults are more likely to visit emergency rooms, be admitted to hospitals, or in some cases, even die.

Exposure to particle pollution may cause people with heart disease to experience chest pain, palpitations, shortness of breath, and fatigue. Particle pollution has also been associated with cardiac arrhythmia and heart attacks.

When exposed to high levels of particle pollution, people with existing lung disease may not be able to breathe as deeply or vigorously as they normally would. They may experience symptoms such as coughing and shortness of breath. Healthy people also may experience these effects, although they are unlikely to experience more serious effects.

Particle pollution also can increase susceptibility to respiratory infections and can aggravate existing respiratory diseases, such as asthma and chronic bronchitis, causing more use of medication and more doctor visits.

Green -- Good
(0 - 50) None

Yellow -- Moderate
(51 - 100) Unusually sensitive people should consider reducing prolonged or heavy exertion.

Orange -- Unhealthy for Sensitive Groups
(101 - 150) The following groups should reduce prolonged or heavy outdoor exertion:
- People with heart or lung disease
- Children and older adults

Red -- Unhealthy
(151 - 200) The following groups should avoid prolonged or heavy exertion:
- People with heart or lung disease
- Children and older adults
Everyone else should reduce prolonged or heavy exertion.

Purple -- Very Unhealthy
(201 - 300) The following groups should avoid all physical activity outdoors:
- People with heart or lung disease
- Children and older adults
Everyone else should avoid prolonged or heavy exertion.

Brown -- Hazardous
(over 300) Remain indoors and limit your time outside. Remove yourself from the area if possible. If you have to go out, wear either the correct cartidge respirator or a particulate respirator (looks like a dust mask but has the word "NIOSH" and either "N95" or "P100" printed on it). More mask information from the CA Dept of Public Health is at http://www.bepreparedcalifornia.ca.gov/Documents/Protect%20Your%20Lungs%20Respirator.pdf