Severity Levels and Exposure Limits

Introduction

Arpuro provides real-time dust monitoring to help organizations understand and manage dust-related risks. Dust levels can change quickly, so tracking conditions regularly and following recommended guidelines is essential. The following information explains how Arpuro classifies dust conditions using severity levels, regulatory limits, and short-term thresholds.

Severity Levels

Arpuro uses color-coded severity levels to help you recognize dust risks at a glance. These labels indicate when conditions are relatively safe and when you may need to take extra precautions:

• Low (Green) The dust concentration is below the 8-hour limit. Prolonged periods at this exposure will not lead to a breach of compliance.
• Moderate (Yellow) The dust concentration is above the 8-hour limit but below the 15-minute limit. Prolonged time within this range may result in a breach of compliance.
• Severe (Red) The dust concentration is above the 15-minute limit but below the 15-second limit*. Caution is advised, as even brief exposure can pose health risks.
• Critical (Purple) The dust concentration is above the 15-second limit*. Caution is necessary; immediate intervention or protective measures are recommended.

* The 15-second limit is not a part of any official regulatory limits but is derived from Haber's law. It corresponds to the dust concentration at a 15-second exposure time. See "Exposure Time" below for more information.

Understanding Exposure Limits

Distinct dust types carry different health risks, resulting in varying exposure limits across locations. Therefore, identical concentrations can represent different risk levels, depending on which regulations apply. Key reminders:

• Staying below specific concentrations does not guarantee safety.
• Any dust exposure inherently involves some risk.
• Each organization must define its own internal dust limits, reflecting the types of dust employees may encounter.
• Arpuro's exposure estimates are broad guidance; short-term high-concentration effects are not precisely documented in current scientific literature.

8-Hour Exposure Limits

Most governments set legal 8-hour exposure limits (especially for respirable and total dust fractions). These limits generally reflect the acceptable concentration level for a typical working lifetime without significant health effects.

Here you can find the list of legal exposure limits for different substances for some jurisdictions:

• Norway: Liste over grenseverdier for forurensninger i arbeidsatmosfæren (Vedlegg 1 til Forskrift om tiltaksverdier og grenseverdier for fysiske og kjemiske faktorer i arbeidsmiljøet samt smitterisikogrupper for biologiske faktorer)
• All countries: German database of legal limits around the world: GESTIS International limit values (Caution: Might contain outdated/inaccurate values.)

Many organizations adopt internal thresholds lower than their jurisdiction's legal limit—often one-quarter (25%)—to provide an extra margin of safety.

Important: Arpuro PM01V00LTE and most optical dust meters do not precisely measure total dust fractions. Arpuro estimates total suspended particles (TSP) as those smaller than about 10–20 µm, which may differ from the total dust fraction defined by local regulations. However, respirable particles typically carry the highest health risks.

15-Minute, Short-Term, and Peak Limits

Many jurisdictions do not explicitly define short-term exposure limits (STELs), often considered 15-minute averages, because of limited scientific data on the health risk of high short-term exposure. Where such limits exist, they are commonly set at two to three times the corresponding 8-hour limit.

For example, the Norwegian Labour Inspection Authority recommends specific exceedance factors for up to 15-minute exposures:

If no explicit 15-minute limit is set, Arpuro will follow these recommendations. If only the 15-minute limit is set, the corresponding 8-hour limit will be used.

Exposure Time

Exposure time in Arpuro provides an estimate of how long individuals can remain in an environment based on the current dust concentration and the exposure limits. It is calculated from a generalized form of Haber's law, used in toxicology to estimate dose-response relationships.

$$C^n \times T = k$$

where the value, n, and the constant, k, are set so that

• when T = 480 (8 hours), the exposure limit, C, equals the specified 8-hour limit,
• when T = 15, the exposure limit, C, equals the specified (or recommended, if not specified) 15-minute limit.

Following this, Arpuro computes the corresponding exposure limit for any exposure time, which gives the foundation for severity levels.

The values of n and k depend on the specified 8-hour and 15-minute limits. Assuming an 8-hour limit of 0.9 mg/m³:

• If the 15-minute limit is set to 2.7 mg/m³ (3x, which is a standard precautionary approach), n and k are approximately 3.15 and 340, respectively.
• If the 15-minute limit is set to 1.8 mg/m³ (2x), n equals 5, and k is approximately 280.