Oregon Department of Environmental Quality

Using visibility to estimate health effects

DEQ monitors air pollution throughout the state to ensure that air quality standards are being met. Since wildfires often occur in remote areas, air monitoring equipment may not be available. Smoke levels can rise and fall rapidly, depending on weather factors including wind direction. Making visual observations using the 5-3-1 visibility index is a simple way of estimating smoke levels and what precautions to take. While this method can be a useful tool, persons should always use caution and avoid going outside if visibility is limited, especially persons who may be sensitive to smoke.

Estimating visibility using the 5-3-1 Index

Determine the limit of your visual range by looking for distant targets or familiar landmarks such as mountains, mesas, hills, or buildings at known distances (miles). The visual range is that point at which these targets are no longer visible. As a general rule of thumb: if you can clearly see the outlines of individual trees on the horizon it is generally less than five miles away. Ideally, the viewing of any distance targets should be made with the sun behind you. Looking into the sun or at an angle increases the ability of sunlight to reflect off of the smoke, and thus making the visibility estimate less reliable.
Once distance has been determined, follow this simple guide:

• If visibility is well over five miles, the air quality is generally good.
• Even if visibility is five miles away but generally hazy, air quality is moderate and beginning to deteriorate, and is generally healthy, except possibly for smoke sensitive persons. The general public should avoid prolonged exposure if conditions are smoky to the point where visibility is closer to the 5-mile range.
• If under five miles, the air quality is unhealthy for young children, adults over age 65, pregnant women, and people with heart and/or lung disease, asthma or other respiratory illness. These people should minimize outdoor activity.
• If under three miles, the air quality is unhealthy for everyone.  Young children, adults over age 65, pregnant women, and people with heart and/or lung disease, asthma or other respiratory illness. These people should minimize outdoor activity.
• If under one mile, the air quality is unhealthy for everyone.  Everyone should avoid all outdoor activities.