Nitrogen dioxide OSHA PEL checker.

The nitrogen dioxide row in Table Z-1

This is the Nitrogen dioxide row of Table Z-1, Limits for Air Contaminants, in 29 CFR 1910.1000, as published in the eCFR. The values are printed exactly as the table prints them.

The ppm column is the working figure for air sampling. The mg/m3 column is the same limit expressed by mass. A reading in ppm is compared with 5, and a reading in mg/m3 is compared with 9. The page does not show what a safe level is. It shows the number OSHA enforces.

Reading the footnotes

Each column heading carries a footnote marker. The eCFR prints them as follows.

• 1: The PELs are 8-hour TWAs unless otherwise noted; a (C) designation denotes a ceiling limit. They are to be determined from breathing-zone air samples.
• (a): Parts of vapor or gas per million parts of contaminated air by volume at 25 °C and 760 torr.
• (b): Milligrams of substance per cubic meter of air. When entry is in this column only, the value is exact; when listed with a ppm entry, it is approximate.
• (c): The CAS number is for information only. Enforcement is based on the substance name. For an entry covering more than one metal compound, measured as the metal, the CAS number for the metal is given—not CAS numbers for the individual compounds.

For this row, the ppm figure is a volume ratio at 25 °C and 760 torr. The 9 mg/m3 figure is its approximate mass equivalent, because the row lists a ppm value as well. Enforcement goes by the substance name in the first column, not the CAS number.

Why this entry is a ceiling

Both numbers in the nitrogen dioxide row start with (C). That makes the 5 ppm a ceiling, not an average. Paragraph (a)(1) of the section reads: “An employee's exposure to any substance in Table Z-1, the exposure limit of which is preceded by a “C”, shall at no time exceed the exposure limit given for that substance. If instantaneous monitoring is not feasible, then the ceiling shall be assessed as a 15-minute time weighted average exposure which shall not be exceeded at any time during the working day.”

Entries without a (C) are 8-hour time weighted averages under paragraph (a)(2). Those allow short peaks if the shift average stays under the limit. A ceiling does not.

Skin designation

The Skin designation column for nitrogen dioxide is blank in the eCFR table.

Worked example

These readings are invented, to show the arithmetic. Suppose instantaneous monitoring is not feasible, so a 15-minute sample is used, and it comes back at 3 ppm. Paragraph (a)(1) compares that with 5 ppm, and 3 ppm is below it.

A 15-minute result of 6 ppm would be above 5 ppm. A low reading earlier in the shift does not offset it. The text says exposure shall at no time exceed the limit, so a ceiling is checked sample by sample, not averaged over the day.

Computation formulae in paragraph (d)

Paragraph (d)(1)(i) gives the cumulative exposure formula for an 8-hour shift: E = (Ca Ta + Cb Tb + . . .Cn Tn) ÷ 8, where E is the equivalent exposure for the working shift, C is the concentration during any period of time T where the concentration remains constant, and T is the duration in hours of the exposure at the concentration C. The value of E shall not exceed the 8-hour time weighted average specified in subpart Z of 29 CFR part 1910 for the substance involved.

Paragraph (d)(2)(i) gives the formula for mixtures: In case of a mixture of air contaminants an employer shall compute the equivalent exposure as follows: Em = (C1 ÷ L1 + C2 ÷ L2) + . . .(Cn ÷ Ln), where C is the concentration of a particular contaminant and L is the exposure limit for that substance specified in subpart Z of 29 CFR part 1910. The value of Em shall not exceed unity (1). The paragraph introducing these formulae applies them to substances for which 8-hour time weighted averages are listed.

What paragraph (e) says about controls

This page does not recommend any exposure-control measure. For reference, paragraph (e) of the section is printed as follows: “To achieve compliance with paragraphs (a) through (d) of this section, administrative or engineering controls must first be determined and implemented whenever feasible. When such controls are not feasible to achieve full compliance, protective equipment or any other protective measures shall be used to keep the exposure of employees to air contaminants within the limits prescribed in this section. Any equipment and/or technical measures used for this purpose must be approved for each particular use by a competent industrial hygienist or other technically qualified person. Whenever respirators are used, their use shall comply with 1910.134.”

Limits of this page

This page covers Table Z-1 and the paragraphs of 1910.1000 quoted above. It does not say that an exposure below a printed limit is safe, it does not cover Table Z-2 or Table Z-3 substances, other OSHA substance-specific standards, state-plan rules or recommended limits published by other bodies, and it does not tell you how to sample, how to control exposure or which respirator to use. The checker compares one number you type with the printed figure and applies the “shall not exceed” wording as written, so a reading equal to the limit is not above it. Anything involving a real workplace belongs with a qualified industrial hygienist, your safety data sheet and the OSHA office for your area. The eCFR lists the newest content version of this section as May 30, 2017.

Sources

Sources opened and read November 2, 2026.

• eCFR: 29 CFR 1910.1000, Air contaminants, Table Z-1 and paragraphs (a), (d) and (e) (text requested as of October 29, 2026; read November 2, 2026)
• eCFR versioner API: version history of 29 CFR 1910.1000 (read November 2, 2026)