Silica Exposure Calculator
OSHA's permissible exposure limit (PEL) for respirable crystalline silica is 50 micrograms per cubic meter of air (µg/m³), and its action level is 25 µg/m³, both as an 8-hour time-weighted average (TWA). A lab report gives a result for each sample and how long the pump ran. Type them in below to get the 8-hour average and what that result requires under the construction or the general industry standard.
Work out the 8-hour average
Type in each air sample result from the lab and how many minutes it ran. You get the 8-hour time-weighted average, where it sits against OSHA's action level and limit, and what that result requires next.
- Sum of result × minutes, ÷ 480
- Share of the PEL
- Next monitoring
- Respirator, if controls cannot bring it down
What this result requires
The formula is OSHA's, 29 CFR 1910.1000(d)(1)(i); the limits and duties are 29 CFR 1926.1153 and 1910.1053; respirator classes 29 CFR 1910.134. General information, not legal advice; a state plan state may set its own rule.
How the 8-hour average is worked out
OSHA's formula for a shift's exposure is in 29 CFR 1910.1000(d)(1)(i): E = (CaTa + CbTb + ... + CnTn) ÷ 8, where C is the concentration during a period, T is its length in hours, and E is the equivalent exposure for the shift. The tool does the same sum in minutes and divides by 480. OSHA's own example in that paragraph is 2 hours at 150, 2 hours at 75 and 4 hours at 50, which gives (300 + 150 + 200) ÷ 8 = 81.25. Type those in and the tool gives the same.
Time that was not sampled adds nothing to the sum, so it counts as zero exposure. That is right only if the worker did no silica work in it. If the samples miss a dusty part of the shift, the average will be too low. The formula is written for an 8-hour work shift, so the tool warns when the samples add up to more than 480 minutes rather than guess how to treat a longer one.
Both silica standards define employee exposure as the exposure that would occur if the worker were not using a respirator. A sample taken outside the respirator is the one that counts.
Worked examples
| Samples | 8-hour TWA | Band | Next monitoring | Respirator |
|---|---|---|---|---|
| 62 µg/m³ for 240 min, 18 µg/m³ for 200 min | 38.5 µg/m³ | Action level to PEL | Within 6 months | Not required by the result |
| 160 µg/m³ for 180 min, 20 µg/m³ for 240 min | 70 µg/m³ | Over the PEL | Within 3 months | APF 10 or more |
| 100 µg/m³ for 240 min, nothing else | 50 µg/m³ | At the PEL, not over | Within 6 months | Not required by the result |
| 20 µg/m³ for 480 min, first monitoring | 20 µg/m³ | Below the action level | May stop | Not required |
| 700 µg/m³ for 480 min | 700 µg/m³ | Over the PEL | Within 3 months | APF 25 or more |
A result exactly at 50 is at or below the PEL, so it is in the six-month band, not the three-month one. The standards' wording is "at or above the action level but at or below the PEL" for six months and "above the PEL" for three.
When to monitor again
Both standards let the employer assess exposure one of two ways. Under the performance option, any mix of air monitoring and objective data that accurately characterizes exposure will do, and there is no fixed schedule. Under the scheduled monitoring option, the rules are the ones the tool applies:
- Initial monitoring below the action level: the employer may stop monitoring the workers it represents.
- At or above the action level and at or below the PEL: repeat within six months of the most recent monitoring.
- Above the PEL: repeat within three months.
- A repeat result below the action level: repeat within six months until two results in a row, taken seven or more days apart, are below the action level, then the employer may stop.
Either way, exposure must be reassessed whenever a change in production, process, control equipment, personnel or work practices could reasonably cause new or higher exposure at or above the action level. Samples must be analyzed by a laboratory that follows Appendix A of the standard. Workers or their representatives must be given the chance to watch the monitoring. Silica objective data and when it replaces air monitoring covers the performance option.
Telling workers the result
The deadline differs between the standards. In construction it is within five working days of completing the assessment (1926.1153(d)(2)(vi)). In general industry and maritime it is within 15 working days (1910.1053(d)(6)). Either way the employer notifies each affected worker in writing or posts the results where all of them can see them, and when a result is over the PEL, the notice must describe the corrective action being taken to bring exposure to or below it.
Over the PEL: controls, then respirators
The employer must use engineering and work practice controls to bring exposure to or below the PEL unless they are not feasible. Where feasible controls are not enough, they must still be used to get exposure as low as feasible, with respirators on top. Respirators are required while controls are being installed, for tasks where controls are not feasible, and where all feasible controls still leave exposure over the PEL. In general industry they are also required in regulated areas.
The respirator has to be good enough for the concentration. Under 29 CFR 1910.134 the maximum use concentration (MUC) of a respirator is its assigned protection factor times the exposure limit. With a PEL of 50, an APF 10 half mask covers up to 500 µg/m³, an APF 25 loose-fitting PAPR up to 1,250, and an APF 50 full facepiece up to 2,500. The tool picks the lowest APF whose MUC covers your 8-hour average. A respiratory protection program under 1910.134 is required wherever respirators are.
Regulated areas, general industry only
In general industry and maritime, 1910.1053(e) requires a regulated area wherever exposure is, or can reasonably be expected to be, over the PEL. It must be marked off, with signs at every entrance; access is limited to people authorized and required to be there; and everyone who enters gets a respirator and must wear it. The construction standard has no regulated areas. It asks instead for written procedures to restrict access to work areas, in the exposure control plan.
Medical exams
The trigger is different in each standard. In construction, medical surveillance is owed to each worker required to use a respirator under the standard for 30 or more days a year (1926.1153(h)(1)). In general industry, it is owed to each worker exposed at or above the action level for 30 or more days a year (1910.1053(i)(1)), whatever the respirator. So a plant worker at 30 µg/m³ for 60 days a year is owed exams, and a construction worker at the same level is not, unless a respirator is required. The exams are at no cost to the worker, with a first one within 30 days of assignment and repeats at least every three years.
Construction tasks on Table 1
In construction, a task listed on Table 1 and done with its controls fully and properly in place needs no exposure assessment. The Table 1 checker shows the controls and respirator for each listed task. Every covered employer still needs a written exposure control plan, which the plan builder drafts. The OSHA silica standard explained sets the whole rule out.
General information from the federal standards, not legal advice and not an industrial hygiene assessment. A state with its own OSHA-approved plan can set stricter rules. Have results interpreted by a qualified person before relying on them.
Sources: 29 CFR 1926.1153 (construction), paragraphs (b), (d), (e) and (h); 29 CFR 1910.1053 (general industry and maritime), paragraphs (b) to (g) and (i); 29 CFR 1910.1000(d), computation formulae; 29 CFR 1910.134, assigned protection factors and maximum use concentration; OSHA, Silica, crystalline.