Filter and Cartridge Selection in Plain Terms

Why this matters

A respirator's facepiece decides how much air leaks around the seal. The filter or cartridge decides what happens to the air that goes through it, and that is where most field mistakes live: a particulate filter worn against a solvent vapor, an N-series filter in an oily spray mist, an organic vapor cartridge used until the wearer smells something, a cartridge from one maker twisted onto another maker's mask. Each looks like protection and each lets the agent through. This card decodes the letters and numbers, says what each family stops and does not, and sets out how a change schedule is actually built.

Whether the task needs a respirator at all, and which facepiece class, comes first; see related: How to Decide Whether a Task Needs a Respirator.

Two families that do different jobs

Particulate filters stop particles: dust, fiber, fume, mist, smoke, and droplets carrying microbes. They are mechanical and electrostatic traps, and they do nothing at all for a gas or a vapor, which passes through them like air.

Chemical cartridges (and the larger canisters) stop specific gases and vapors, usually by adsorbing them onto treated activated carbon. They do nothing for particles unless combined with a particulate filter, and each type is approved only for the gases it is labeled for.

So the first question is always the physical form of the agent. Paint spray is both: liquid droplets (particulate) and evaporating solvent (vapor). Welding is fume (particulate) plus gases. Grinding is particulate. A refrigerant leak is a gas no air-purifying cartridge is approved for, and its real danger in a small space is oxygen displacement; see related: Refrigerant Decomposition and Oxygen Displacement.

Particulate filters: the letter and the number

Non-powered particulate filters are certified by NIOSH under 42 CFR Part 84, and the rating is a letter plus a number.

The letter is about oil.

• N: not resistant to oil. Fine for dry dusts and water-based mists, and for anything with no oil aerosol present.
• R: resistant to oil. NIOSH's guidance limits it to a single shift, about 8 hours of use, where oil aerosol is present.
• P: oil-proof. Usable in oil aerosol for the service time the manufacturer states.

Oil matters because oil droplets degrade the electrostatic charge many filter media rely on, so an N filter in an oil mist loses efficiency as it is used. Cutting fluid mist, oil-based spray coatings and some lubricant sprays are the usual field cases.

The number is the minimum efficiency against the most penetrating particle size in NIOSH's test: 95 means at least 95 percent, 99 means 99 percent, and 100 means 99.97 percent. That is filter efficiency, not the protection the whole respirator gives; total protection is capped by the facepiece's assigned protection factor, because air that leaks around the seal never meets the filter. A P100 filter on a half mask is still an APF 10 respirator.

Powered air-purifying respirators use filters rated HE (high efficiency), certified to the same 99.97 percent level under their own approval.

When to change a particulate filter: when breathing gets harder, when it is wet, damaged or visibly soiled, when the R-series 8-hour limit or a P-series manufacturer limit is reached, and under whatever the substance standard says (some require changing at the end of each shift). A particulate filter generally gets more efficient, not less, as it loads; the reason to change it is resistance and condition, not a fading efficiency.

Chemical cartridges: what each type is for

Cartridges are certified for named gas and vapor classes. The common ones in field service, with the color code most makers follow under the ANSI/AIHA Z88.7 convention (confirm on the label, not by color alone):

A combination cartridge pairs a gas or vapor element with a particulate filter, either built in or as a prefilter held by a retainer. Spray work, where both a mist and a vapor are present, is the classic case.

What cartridges cannot do. No air-purifying cartridge supplies oxygen. Common cartridges are not approved for carbon monoxide. A cartridge approved for one gas gives no protection against a different one, even a similar-sounding one. And some agents, isocyanates among them, have such poor warning properties and such low limits that the usual answer is supplied air rather than a cartridge; see related: Isocyanates and Spray Foam in the Homes You Enter.

The approval covers the assembly, not the part

NIOSH approves a respirator as a complete assembly: this facepiece with these cartridges, filters and retainers from the same maker. The approval number on the label begins "TC-" followed by a schedule and a sequence (TC-84A for particulate respirators, TC-23C for chemical cartridge respirators, TC-21C for powered air-purifying). Two consequences:

• Mixing parts voids the approval. A cartridge from one maker on another's facepiece, even if it threads on, is an unapproved respirator.
• Check that it is real. NIOSH maintains an online Certified Equipment List and publishes notices of counterfeit and misrepresented respirators. A filtering facepiece with no approval number printed on it, or one that does not appear on the list, is not an approved respirator.

Under 29 CFR 1910.134(d)(1)(ii), the employer must select a NIOSH-certified respirator and use it in compliance with the conditions of its certification. Construction work reaches the same rule through 1926.103.

READING A COMBINATION CARTRIDGE LABEL
(layout illustrative; read your own)

OV / P100
  OV   -> organic vapor element
  P    -> particulate, oil-proof
  100  -> 99.97 percent filter efficiency

Bands: black (OV) + magenta (P100)

TC-84A-nnnn
  -> NIOSH approval for the WHOLE assembly
     listed on the approval label

"For use with facepiece series ..."
  -> any other facepiece voids approval

Cautions and limitations: letter codes
  -> look them up on the approval label
     (oxygen-deficient air, IDLH, etc.)

Change schedules for cartridges: what the standard requires

A carbon cartridge works until its carbon is saturated, then the vapor comes through. That moment is called breakthrough, and it is not something a person can reliably sense. Odor thresholds vary between people, olfactory fatigue dulls the sense over a shift, and some vapors have no odor at the concentrations that matter.

So 29 CFR 1910.134(d)(3)(iii)(B) requires, for gases and vapors, either a respirator with an end-of-service-life indicator (ESLI) certified by NIOSH for that contaminant, or a change schedule based on objective information or data that will ensure cartridges are changed before the end of their service life. The employer has to describe in the written program the information and data relied on. Smelling it, tasting it or feeling it is a reason to leave the area now; it is not a change schedule.

What sets service life:

• Concentration: higher concentration, shorter life.
• Humidity: water vapor competes for the carbon; high humidity shortens life for many organic vapors.
• Work rate: heavier breathing pulls more air through the carbon per hour.
• Temperature, the boiling point of the vapor (low-boiling vapors are held poorly), and mixtures, where one vapor can displace another already adsorbed.

How to build one. Most cartridge makers publish service-life calculators or tables for their own cartridges: you enter the chemical, the estimated concentration, humidity, temperature and work rate, and they return an estimated service life. That output, with the inputs recorded, is objective information. OSHA has also published rules of thumb for screening organic vapors (for example, that cutting the concentration tenfold extends service life about fivefold, and that humidity above about 85 percent roughly halves it); those are screens for checking a calculator's answer, not a schedule on their own.

One more limit. An opened organic vapor cartridge keeps adsorbing whatever is in the air while it sits on the truck, and adsorbed low-boiling vapors can migrate through the carbon overnight and release on the next donning. Many makers direct that a cartridge used against volatile organics be discarded at the end of the shift regardless of the hours left. Follow the maker's instruction for your cartridge.

Worked example: spraying an oil-based primer in a basement

A painting and handyman crew sprays a solvent-borne stain-blocking primer on a basement ceiling, about three hours of spraying with an airless sprayer. Use is required under the shop's program, since the solvent concentration in an enclosed basement during spraying is expected to exceed the limit without respiratory protection, and the crew is medically cleared and fit tested on the half masks they will wear.

Form of the agent: two at once. Droplets of primer (particulate, carrying an oily resin) and mineral-spirit-type solvent vapor.

Particulate choice: oil is present, so N is out. P is preferred over R because the job may run longer than one shift; P100 is chosen.

Vapor choice: organic vapor. Result: an OV/P100 combination from the same maker as the facepiece.

Change schedule: the foreman uses the cartridge maker's calculator with the SDS's main solvent, an estimated concentration from the maker's guidance for spray application in an enclosed room, 70 percent relative humidity measured in the basement, and a heavy work rate. Say, illustratively, the calculator returns about 4 hours of service life at those inputs. The shop sets a change at every 2 hours of spraying, half the estimate to allow for the uncertainty in the concentration input, plus discard at the end of the shift. Three hours of spraying therefore uses two sets: one changed at 2 hours, the second discarded at the end of the shift after 1 hour. The inputs, the calculator's answer and the 2-hour interval go into the written program next to this task.

The failure this prevents: the crew runs one set all day "until it smells," and the solvent comes through at the hour the basement is hottest and the sprayer busiest, which is also the hour noses have stopped noticing.

What changes the answer: if the primer were water-based, the particulate could drop to N-series, though vapors from coalescing solvents may still call for OV, per the SDS. If ventilation brings the concentration down tenfold, the screen above suggests roughly five times the service life, and the shop could lengthen the interval, but only after re-running the calculator with the new input, never by applying the rule of thumb alone.

References

• OSHA, 29 CFR 1910.134(d)(1)(ii) certified respirators and (d)(3)(iii) gas and vapor change schedules; 29 CFR 1926.103 for construction
• NIOSH, 42 CFR Part 84 respirator certification and the NIOSH Certified Equipment List
• ANSI/AIHA Z88.7, color coding of air-purifying respirator canisters, cartridges and filters
• See related: How to Decide Whether a Task Needs a Respirator
• See related: How to Do a User Seal Check Every Time