Measurement of chemical substances in workplace air: what determines the scope of measurement?

Why a general brief to "measure chemicals" is not enough

Measurement of chemical substances in workplace air is among the common documentation for job categorisation, workplace environment inspection, requests from the regional hygiene station (KHS) or verification of ventilation and extraction effectiveness. In practice it applies to paint shops, bonding, degreasing, welding, plastics processing, chemical surface treatment, laboratories, mixing rooms, pressing operations, operations with material heating or workplaces where cleaning and process agents are used.

The scope of measurement cannot be determined only from the fact that "chemicals are used" in the operation. It is important to find out which substances may actually be present in the air, at what stage of work they arise, how long the worker is exposed and whether it is a normal state, peak operation or extraordinary activity.

The aim of measurement is not to capture a random moment in operation, but to assess professionally the employee's actual inhalation exposure.

What determines the scope of measurement

The scope of measurement consists of several professional decisions. It is necessary to determine which substances will be measured, where samples will be taken, how long sampling will last and how results will be evaluated against hygiene limits.

A proper measurement design therefore starts with analysis of the operation. Only then does it make sense to determine specific substances, number of samples, type of sampling and measurement duration.

Safety data sheets as the starting point

Safety data sheets are the basic documentation for measurement design. They contain information on hazardous components of mixtures, classification, physico-chemical properties, possible exposure and recommended protective measures.

From safety data sheets the following are usually assessed:

• composition of the mixture and listed hazardous substances,
• volatility of components,
• hazard classification,
• possible irritant, sensitising, toxic or carcinogenic effects,
• recommended technical and personal protective equipment,
• exposure limit information where stated,
• method of product use in the specific technology.

A safety data sheet alone is not enough. It gives information about the substance or mixture, not about actual exposure in the given operation. The same product can lead to very different concentrations depending on whether it is applied manually with a brush, sprayed, heated, used in enclosed equipment or applied in a poorly ventilated space.

Practically important: the safety data sheet helps determine what may be a problem. Measurement then verifies how significant that problem actually is in the specific operation.

Work procedure: when a substance really arises or escapes

For chemical substances it is necessary to know the actual work procedure. It is not enough to know that, for example, adhesive, thinner, cleaning agent or paint is used. What matters is how the product is handled.

An employee may have different exposure in these situations:

1. opening packaging and dosing the product,
2. manual application,
3. spraying,
4. mixing mixtures,
5. cleaning tools,
6. drying or curing,
7. heating material,
8. fault or non-standard condition,
9. maintenance and cleaning of technology.

In some operations the highest exposure does not occur during the main production operation, but during preparation, cleaning or waste handling. For example, in a paint shop significant exposure may arise not only from coating application itself, but also from colour mixing and cleaning of spray equipment. In bonding, the method of application and ventilation time may be decisive. In plastics processing, process temperature and possible thermal decomposition products may be important.

Shift length and exposure duration

Shift length is essential for evaluation of results. Hygiene limits such as PEL are based on a full-shift time-weighted average. That means not only instantaneous concentration is evaluated, but exposure over time.

If the worker performs a risky activity for the whole shift, the situation is simpler. If they work with a chemical substance only part of the shift and perform other work for the rest of the time, the time course of exposure must be taken into account.

Simple example

A worker may have the following regime during a shift:

In such a case it is necessary to decide whether the whole shift, a selected exposure period, a short-term peak or a combination of these approaches should be measured. The result is then evaluated so that it corresponds to the worker's actual exposure.

PEL and NPK-P: two different views of the limit

When evaluating chemical substances in workplace air, two basic limit values are used in particular: PEL and NPK-P.

PEL corresponds to a longer-term view of the work shift. It is important where the employee works with a substance repeatedly or for a long time.

NPK-P is important for substances that may briefly irritate eyes or airways, act acutely or create short-term peaks during certain operations. In practice this may involve opening containers, cleaning, pouring, spraying, emergency ventilation or short intensive technological steps.

In simplified terms: PEL answers the question "what is the average exposure over the shift". NPK-P answers the question "whether unacceptable short-term peaks occur in operation".

TWA: time-weighted average

TWA means time-weighted average. In Czech practice it is used mainly to calculate full-shift exposure, where individual concentrations are weighted according to the duration of given activities.

If exposure during the shift is variable, one cannot look only at a single measured value without time context. A short high concentration may be significant for NPK-P, but for the full-shift average it has a different weight than exposure lasting several hours.

Orientational calculation principle:

TWA = (C1 × t1 + C2 × t2 + C3 × t3 + ... ) / total time

Where:

• C is substance concentration,
• t is duration of the given activity,
• the result is the time-weighted average for the evaluated period.

This principle is important for operations where the worker alternates work at the emission source, handling away from the source and administrative or control activities.

Personal and stationary sampling

Personal or stationary sampling is used when measuring chemical substances. Both have their place, but they do not answer the same question.

Personal sampling is carried out so that the sample best corresponds to the air the worker actually breathes. It is therefore essential when evaluating employee exposure and job categorisation.

Stationary sampling is useful, for example, to verify concentration at a source, in the workplace space, near technology or to compare different locations. It does not automatically replace personal sampling, because the worker may move, change position and perform different activities.

If the purpose of measurement is evaluation of employee exposure, the sampling design must focus primarily on the worker's breathing zone and actual work regime.

Selection of a representative shift

A representative shift is one that corresponds to normal and substantively significant operating state. It should not be an artificially created extreme, nor a day when risky work is barely performed.

When selecting a shift the following are assessed in particular:

• whether normal production is running,
• whether usual chemical substances and mixtures are used,
• whether technology is operated at standard output,
• whether normal ventilation and extraction equipment are running,
• whether workers perform typical activities,
• whether it is not an extraordinary regime, shutdown or trial operation,
• whether the shift includes activities with the most significant exposure.

In some operations it may be appropriate to measure more than one shift or more work regimes. Typically where different products, different chemical agents, different recipes or different application methods alternate.

Example: paint shop

In a paint shop exposure may differ during priming, top coating, gun cleaning and colour mixing. If only a calmer regime without normal application is measured, the result may not be a suitable basis for categorisation.

Example: bonding

In bonding the type of adhesive, application method, application area, time of opening packaging, temperature, ventilation intensity and number of bonded pieces per shift may matter. Two workplaces using the same product may therefore not have the same exposure.

Example: cleaning and degreasing

Cleaning may create briefly high exposure even when the activity itself does not last long. In such a case full-shift evaluation may be important, but also assessment of short-term concentrations in relation to NPK-P.

Mixtures of chemical substances and additive effect

In practice the employee is often exposed not to a single substance. In paint shops, bonding, degreasing or cleaning, a mixture of organic substances may be present in the air. If substances act on the same organ system or have a similar effect, their combined action is also evaluated.

This is especially important for solvent mixtures. Individual substances may be below their limit separately, but their sum in relation to limit values may be significant for evaluation.

In practical terms: it is not always correct to say that "each substance is below the limit, so everything is fine". For mixtures it is necessary to assess combined effect according to the applicable evaluation rules.

What to prepare before measurement

Good preparation saves time and reduces the risk that an important substance or activity is overlooked. Before measurement it is advisable to prepare documentation describing real operation.

Recommended documentation:

1. safety data sheets of substances and mixtures used,
2. list of chemical agents used,
3. description of technology and work procedures,
4. information on shift length and breaks,
5. time snapshot of work activities,
6. number of workers and description of job positions,
7. data on ventilation and local extraction,
8. information on use of respirators and other PPE,
9. existing job categorisation if any,
10. KHS request if issued.

The more accurate the input documentation, the better it is possible to determine whether, for example, full-shift exposure, short-term peaks, a specific substance, group of substances or several work regimes should be measured.

Most common mistakes when measuring chemical substances

Mistakes often arise already in the brief. The operator orders general measurement without sufficient description of activities and products used. The result may be a report containing measured values that does not precisely answer the KHS question, categorisation or actual work risk.

Common mistakes include:

• measurement without current safety data sheets,
• selection of substances only from product name,
• omission of cleaning and auxiliary operations,
• measurement on a day with low production output,
• substituting stationary sampling for personal exposure evaluation,
• missing time snapshot of work,
• failure to evaluate short-term peaks,
• failure to take into account mixtures of substances with similar effect,
• unclear distinction of job positions,
• using results for a different operating regime than the one in which measurement was carried out.

Properly designed measurement should be defensible not only technically but also substantively. It must be clear why exactly those substances, those workplaces, those workers and that measurement day were selected.

How measurement helps the operator

Measurement of chemical substances is not just formal documentation for the authority. Well-conducted measurement can show whether the workplace environment is safely below limits, whether ventilation needs improvement, work procedure adjustment, change of chemical agent or supplementary protective measures.

Results can be used for example for:

• job categorisation,
• documentation of KHS request,
• verification of extraction effectiveness,
• comparison of old and new technology,
• assessment of change of chemical agent,
• proposal of occupational health measures,
• internal workplace environment control,
• decisions on need for personal protective equipment.

In many operations the greatest benefit of measurement is that it separates assumptions from actual data. The operator then knows whether the problem is significant, marginal or concerns only a specific activity or shift.

Summary

The scope of measurement of chemical substances in workplace air is determined by a combination of several factors: safety data sheets, work procedure, shift length, exposure duration, PEL and NPK-P limit values, sampling method and selection of a representative shift.

For evaluation of employee exposure, personal sampling in the breathing zone and correct evaluation of time-weighted average are essential. Stationary sampling has its place when evaluating space, sources or ventilation effectiveness, but does not automatically replace personal exposure of the worker.

Send us safety data sheets of products used, a description of technology, work procedure, shift length and any KHS request. We will propose a suitable scope of measurement of chemical substances in workplace air so that results are usable for job categorisation, workplace environment inspection and further dealings with the hygiene station.

Factual basis of the article

The article is based mainly on the following regulations:

• Government Regulation No. 361/2007 Coll., laying down conditions for health protection at work,
• Decree No. 432/2003 Coll., on job categorisation,
• Act No. 258/2000 Coll., on public health protection.

Government Regulation No. 361/2007 Coll. defines PEL as full-shift time-weighted average and NPK-P as maximum permissible concentration for short-term evaluation. It also states that inhalation exposure relates to measurement of chemical substances or dusts in the employee's breathing zone. Decree No. 432/2003 Coll. follows on job categorisation and for chemical substances works with measured concentrations in workplace air, their comparison with PEL and NPK-P values and evaluation of substance mixtures with possible additive effect.