Air filter with visual filter life indicator zone and sorbent-loaded visual reference zone

1. An air filter comprising an air filter media that comprises a visual filter life indicator zone and a visual reference zone, the air filter media comprising:

a nonwoven fibrous filtration web comprising at least some fibers that comprise charged electret moities, wherein the nonwoven fibrous filtration web comprises a first major surface with an active particle-filtration area that includes at least one sorbent-free area and at least one sorbent-loaded area,

wherein the at least one sorbent-free area is configured to provide a visual filter life indicator zone of the air filter, and

wherein the at least one sorbent-loaded area is configured to provide a visual reference zone for the visual filter life indicator zone.

2. The air filter of claim 1 wherein in the sorbent-loaded area, sorbent particles are adhesively bonded to fiber portions that provide the first major surface of the nonwoven fibrous filtration web.

3. The air filter of claim 1 wherein the at least one sorbent-loaded area occupies at least about 50 percent of the active particle-filtration area of the first major surface of the nonwoven fibrous filtration web, and wherein the at least one sorbent-free area occupies at most about 50 percent of the active particle-filtration area of the first major surface of the nonwoven fibrous filtration web.

4. The air filter of claim 1 wherein the at least one sorbent-loaded area occupies at least about 70 percent of the active particle-filtration area of the first major surface of the nonwoven fibrous filtration web, and wherein the at least one sorbent-free area occupies at most about 30 percent of the active particle-filtration area of the first major surface of the nonwoven fibrous filtration web.

5. The air filter of claim 1 wherein in the at least one sorbent-loaded area, sorbent particles are present on the first major surface of the nonwoven fibrous filtration web at a loading of at least about 60 grams per square meter, and wherein in the at least one sorbent-free area, sorbent particles are present on the first major surface of the nonwoven fibrous filtration web at a loading of at most about 5 grams per square meter.

6. The air filter of claim 1 wherein in the at least one sorbent-loaded area, sorbent particles are present on the first major surface of the nonwoven fibrous filtration web at a loading of at least about 80 grams per square meter, and wherein in the at least one sorbent-free area, sorbent particles are present on the first major surface of the nonwoven fibrous filtration web at a loading of at most about 1 gram per square meter.

7. The air filter of claim 1 wherein the sorbent-loaded area comprises primary sorbent particles that are mixed with one or more types of secondary sorbent particles.

8. The air filter of claim 1 wherein in the at least one sorbent-loaded area, sorbent particles are present that comprise at least some of activated carbon particles.

9. The air filter of claim 8 wherein the activated carbon particles exhibit a mesh size that is from about 20 mesh to about 100 mesh.

10. The air filter of claim 1 wherein the at least one sorbent-free area is provided in the form of a plurality of discrete sorbent-free areas.

11. The air filter of claim 1 wherein the at least one sorbent-loaded area is provided in the form of a plurality of discrete sorbent-loaded areas.

12. The air filter of claim 1 wherein the nonwoven fibrous filtration web exhibits a particle Filtration Efficiency of at least about 50%.

13. The air filter of claim 1 wherein the at least one sorbent-free area of the nonwoven fibrous filtration web comprises at least one region that is a passivated region that is configured to provide a visual baseline zone of the air filter.

14. The air filter of claim 1 wherein the air filter is a high bypass ratio air filter comprising a conformable, unframed air filter media that is configured to be installed on a portion of an upstream face of a filter-support layer of an air-handling system, in a high bypass ratio configuration in which the air filter media occupies less than 85% of a nominal air-transmissive area of the filter-support layer.

15. The air filter of claim 1 wherein the air filter media is conformable into an arcuate shape so that the filter media can be installed on an upstream face of an arcuate filter-support layer of an air-handling system.

16. The air filter of claim 1 wherein the filter media is pleated and wherein a perimeter support frame is mounted to a perimeter of the filter media.

17. The air filter of claim 1 wherein the filter media is unpleated.

18. The air filter of claim 1 further comprising a nonwoven fibrous cover web that is affixed to the nonwoven fibrous filtration web so that sorbent particles of the at least one sorbent-loaded area of the first major surface of the nonwoven fibrous filtration web are present as a layer that is sandwiched between the first major surface of the nonwoven fibrous filtration web, and a major surface of the cover web, wherein the cover web exhibits an area density of less than about 20 grams per square meter and a thickness of less than about 1 mm and is at least substantially impenetrable by the sorbent particles.

19. The air filter of claim 1 wherein at least some fibers of the nonwoven fibrous filtration web are comprised of melt-processable polymeric resin that includes a white pigment.

20. A method of installing, visually monitoring, and replacing an air filter, the method comprising:

a) installing the air filter of claim 1 in an air-handling system;

b) periodically visually inspecting the air filter so as to ascertain how closely a visual appearance of the at least one sorbent-free area approaches a visual appearance of the at least one sorbent-loaded area;

optionally repeating step b) one or more additional times;

and,

c) when the visual appearance of the at least one sorbent-free area is observed to approach the visual appearance of the at least one sorbent-loaded area so that a CIE2000 L*a*b* color space L* axis ΔE of the at least one sorbent-free area relative to the at least one sorbent-loaded area is less than 30, removing the air filter from the air-handling system.