Characterization of occupational inhalation exposures to particulate and gaseous straight and water-based metalworking fluids

Abstract

Exposure assessments to metalworking fluids (MWF) is difficult considering the complex nature of MWF. This study describes a comprehensive exposure assessment to straight and water-based MWFs among workers from 20 workshops. Metal and organic carbon (OC) content in new and used MWF were determined. Full-shift air samples of inhalable particulate and gaseous fraction were collected and analysed gravimetrically and for metals, OC, and aldehydes. Exposure determinants were ascertained through observations and interviews with workers. Determinants associated with personal inhalable particulate and gaseous fractions were systematically identified using mixed models. Similar inhalable particle exposure was observed for straight and water-based MWFs (64–386 µg/m³). The gaseous fraction was the most important contributor to the total mass fraction for both straight (322–2362 µg/m³) and water-based MWFs (101–699 µg/m³). The aerosolized particles exhibited low metal content irrespective of the MWF type; however, notable concentrations were observed in the sumps potentially reaching hazardous concentrations. Job activity clusters were important determinants for both exposure to particulate and gaseous fractions from straight MWF. Current machine enclosures remain an efficient determinant to reduce particulate MWF but were inefficient for the gaseous fraction. Properly managed water-based MWF meaning no recycling and no contamination from hydraulic fluids minimizes gaseous exposure. Workshop temperature also influenced the mass fractions. These findings suggest that exposures may be improved with control measures that reduce the gaseous fraction and proper management of MWF.

Introduction

Exposure to aerosols from metalworking fluids (MWF) has been related to a series of adverse health outcomes^1,2. Chronic or acute exposures to MWF have been reported to induce respiratory diseases¹, skin problems³ and cancers⁴. Occupational asthma or outbreaks of hypersensitivity pneumonitis^5–7 are respiratory diseases often mentioned, possibly resulting from oxidative stress⁸ and immune-mediated reactions. As many as 10% of metalworkers were presented with skin symptoms in two population-based epidemiological studies³. In Switzerland, a total of 1280 work-related skin diseases were attributed to MWF between 2004 and 2013 and 96 to respiratory diseases⁹. A recent study on MWF exposures and cancer outcomes indicated that even at low exposures (0.1 mg/m³), a non-negligible (> 3%) excess lifetime risks of cancer persisted². The primary affected cancer sites in men were the larynx, esophagus² and bladder¹⁰. This is the reason why occupational exposure levels (OELs) have been edicted for the inhalable particulate fraction, e.g., the OEL in France is 0.5 mg/m³ and 1 mg/m³ in Switzerland.

This inconsistency in health effects might be related to the lack of comprehensive exposure assessments considering the complex nature of MWF aerosols. MWF are used in metal machining operations to lubricate, reduce heat and friction, remove metal particle parts, and increase the tool life. MWF are complex mixtures with the base substance being a straight oil, a natural esterified oil or a completely synthetic compound, which may or may not be miscible in water. MWF are classified based on the base oil into straight, water-soluble, semi-synthetic or completely synthetic fluids. Different additives for emulsifying, antiwear, lubricity, antimisting, corrosion inhibitor or antibacterial (for aqueous MWF) are added to optimize MWF properties for its purpose¹¹. In addition, secondary components are formed in situ during use¹². Development of bacteria and fungi in water-based MWF are also associated with respiratory outbreaks⁶. Finally, MWF formulations and machine designs are constantly changing due to legal constraints (e.g., ban of formaldehyde releasing MWF) or developments for new applications (e.g., nanofluids)¹³. All these changes can alter workers’ exposures over time¹⁴.

Aerosols consisting of particulate matter (droplets of MWF, metals) and vapour (volatile organic compounds) are generated during machining due to the high speed of moving parts and liquid sprays¹¹. Common methods for measuring airborne MWF is personal air sampling collecting MWF on a filter followed by gravimetric analysis to quantify the particulate mass concentration in air. However, this method is subject to loss because the volatile or semi-volatile MWF fraction can evaporate from the filter¹⁵ and thus, does not account for exposures to the gaseous fraction. Consequently, exposures are not properly characterized. For a comprehensive assessment of MWF exposure, both fractions of the MWF aerosols often referred to as oil mist, should be collected and quantified separately to implement appropriate risk reduction strategies¹⁶. In addition to the total MWF mass, other component-specific exposure assessments are bioaerosols^6,17, endotoxins¹⁸, and alkanolamines¹⁹. Due to the chemical complexity of the MWF aerosols, component-specific approaches are tedious calling for new integrative strategies^20,21.

Exposure determinants²² for oil mist exposures have been reported in numerous studies^23,24 and several are associated with the particulate MWF mass (reviewed by Ref.¹⁴). The exposure determinants most often cited are fluid and operation type, machine characteristics including age^24–26, cutting speed²⁷, engineering controls including machine enclosures and local exhaust ventilation systems^{19,24,26,28,29}, and MWF maintenance²⁵.

We conducted a comprehensive exposure assessment for straight and water-based MWF. Our aims were to:

1. Characterise metal and organic carbon (OC) content in new and used MWF fluids.

2. Describe worker’s exposures to inhalable and gaseous MWF mass fractions including OC and metal content in the particulate fraction as well as aldehydes in the gaseous fraction.

3. Identify important exposure determinants for inhalable particulate and gaseous mass fractions.

This exposure characterisation will be important when associating MWF exposures to health effects in an ongoing epidemiological study³⁰.

Materials and methods

Companies

Companies using MWF for metal machining were contacted with the help of national or local occupational networks. A total of 16 companies (10 in France and 6 in Switzerland) were willing to participate. The industrial sectors and companies investigated were different between countries. Swiss companies (3–350 employees) produced watch and medical components as well as electrical contactors using only straight oil whereas the sectors of activity for the French companies (40–2500 employees) were more varied: automotive supplies, industrial mechanics, machine tool manufacturing, aeronautical and space constructions. Water-based MWF were systematically used in the French companies except for two sites, which also used straight oils in some machines. This selection of companies was not intended to be representative but rather reflects the wide variability of situations and exposures within each sector considered.

Exposure measurements

A 2-day sampling campaign (Monday–Tuesday) in each workshop was conducted between February 2018 and June 2019. Full-shift (7–8 h) personal air samples were collected from two or three MWF exposed operators in each workshop. As this study belongs to a larger epidemiological field research on early effect markers among metal industry workers³⁰, we included at least one reference worker not exposed to MWF from the same company. Reference participants were mainly in administrative jobs but sometimes in quality control or assembly departments.

The personal sampling train for collecting the particle and gaseous MWF mass consisted of a PTFE filter (25 mm EMFAB TX 40H120WW, VWR, Switzerland) in an open-faced IOM filter cassette attached to a XAD-2 sorbent tube connected to a personal pump operating at a flow rate of 2 L/min. Personal and stationary sampling for particulate MWF using a quartz filter (37 mm, Fiorini, France) in an open-faced IOM filter cassette was also collected with the same flow rate and analysed for OC content. Two to three stationary sampling points (at approximately 1.5 m high) in the vicinity of the participating workers’ workspace were also monitored for metals, aldehydes and OC. For metal analysis, the particulate fraction was collected on a cellulose ester filter (Accu-CAP Capsule SKC, Blanc Labo, Lonay, Switzerland) inserted into a closed side cassette of 37 mm connected to an individual sampling pump set at a sample flow rate of 2 L/min. Aldehydes were collected with DNPH-coated silicagel tubes (Lp-DNPH S10, Merck, Switzerland) at 1 L/min, as described in Sauvain²¹. Both new and used MWF were obtained from each surveyed shop.

Analysis

The inhalable MWF mass was quantified gravimetrically (Metropol-M 282³¹, adapted from the NMAM 5524 (NIOSH)) with a limit of detection (LOD) of 20 µg/m³ and a limit of quantification (LOQ) of 60 µg/m³. OC was measured as described in detail elsewhere²¹. Briefly, a fraction of the quartz filter was introduced in a thermo-optical carbon analyzer (Sunset Laboratory, Tigard, Oregon, US) and heated under an inert atmosphere (He, temperature steps until 650 °C) and finally under oxygen (at 650 °C, to determine the “elemental” carbon). The thermal stability of compounds in the MWF aerosol were tested at five different temperatures (130 °C; 250 °C, 350 °C, 450 °C, 650 °C). The LOD was 4 µg/m³ with a coefficient of variation (CV) of 20%.

The metal content in the new and used MWF as well as in the particulate aerosol fraction were quantified by digesting 500 mg MWF (or the cellulose filter) in a mixture of 4 mL HNO₃ 65% (Analytical Reagent Grade, Fluka), 1 mL H₂O₂ 30% (ACS Reagent, 30% Sigma Aldrich) and 5 mL ultrapure water and mineralized with a heated microwave (Multiwave 7000 Anton Paar) followed by 1 mL HCl after mixture returned to room temperature. Selected metals (Al, B, Ca, Cu, Fe, Mn, Ni, Pb, W, Zn) were quantified using an optical emission spectrometer (ICP-OES, Agilent 5100) with multi-element calibration. The metal selection was based on the alloy compositions usually machined in the selected industries (Table 1). The LOD was 0.04–3.2 µg/m³ depending on the metal.

Table 1.

Characteristics of the different workshops considered in this studyusing MWF in Switzerland (S) and France (F).

Companies	Workshop	Metal machined	Number and type of machines	Type of MWF	Machining operation
1 (S)	1	Aluminium, alloyed steel	5 CNC	One straight	Turning, drilling, milling
2 (S)	2	Brass, copper, bronze	220 CAM	One straight	Turning, drilling, milling
2 (S)	3	Brass, copper, bronze	37 CNC	One straight	Deburring
3 (S)	4	Alloyed steel	98 CNC/CAM	One straight	Turning, drilling, milling
3 (S)	5	Alloyed steel, titanium	43 CNC	Two straights	Turning, drilling, milling
4 (S)	6	Brass, bronze	106 CAM	One straight	Turning, drilling
4 (S)	7	Brass, bronze	72 CAM	One straight	Deburring, stamping
5 (S)	8	Titanium, maillechort	13 CNC	One straight	Turning, drilling, milling
6 (S)	9	Alloyed steel, brass, copper	106 CNC/CAM	Four straights	Turning, drilling, milling
6 (S)	10	Alloyed steel, brass, copper, maillechort	45 CNC/CAM	Four straights	Turning, drilling, milling
7 (F)	11	Stainless steel, aluminum	36 Stamping press	Two aqueous and two straights	Stamping
8 (F)	12	Aluminum, titanium	42 CNC	One aqueous and two straights	Drilling, milling, deburring
9 (F)	13	Aluminum, titanium, Inconel	22 Grinding wheels, sawing machine	Three aqueous	Drilling, milling
10 (F)	14	Copper, nickel	18 CNC	Three aqueous	Grinding
11 (F)	15	Stainless steel	30 CNC	One aqueous	Drilling, milling, deburring
12 (F)	16	Stainless steel	45 CNC/grinding wheels	Three aqueous	Drilling, milling, grinding
13 (F)	17	Stainless steel	36 Stamping press	Two aqueous and two straights	Stamping
14 (F)	18	Aluminum	54 CNC/CAM	Three aqueous	Grinding
15 (F)	19	Stainless steel, aluminum, Inconel	16 CNC	Three aqueous	Drilling, milling, deburring
16 (F)	20	Aluminum	9 CNC/grinding wheels	One aqueous	Drilling, milling, deburring

S Switzerland, CNC Computer numerical control, CAM camshaft machines, F France.

Fifteen gaseous aldehydes were considered in this study (formaldehyde, acetaldehyde, acetone, acrolein, propanal, crotonaldehyde, butyraldehyde, benzaldehyde, isovaleraldehyde, valeraldehyde, o-tolualdehyde, m + p-tolualdehyde, glutaradlehyde, hexanal, 2,5-methylbenzaldehyde). The DNPH cartridge was eluted with 3 mL acetonitrile and the eluate analysed by LC-UV chromatography, as described in Ref.²¹. The LODs for the aldehydes were 0.12–0.9 µg/m³ with CV of 7%.

Exposure determinants

A sampling sheet listing the selected exposure determinants was used in all companies after it had been tested in preliminary field visits. An explicit focus was to identify major factors influencing the generation of airborne particulate and gaseous MWF fractions using an a priori selection of determinants based on the scientific literature^19,26,32. Four main categories of determinants were considered (Supplementary material, Fig. S1):

1. MWF was water or oil based. This category included any information on the MWF (viscosity) and the management of MWFs (treatment, duration before change, day of the most recent change) as well as the presence of solvents in the shops.

2. Tasks were assessed on the number of machines each worker had to manage and the daily temporal percentage spent on nine tasks: machining (process supervision); settings (machine set-up, programming); switching tool (including tool sharpening); pieces finishing (including cleaning with a solvent and/or compressed air); machine cleaning; machine maintenance (including MWF and chips management); handling (pieces packaging and storage including logistics); displacement (more than 10 m away from workplace) and others (quality control, administrative work).

3. Process description included type of machine, type of metal machined, machine enclosure and whether it was connected to an air purification system. Age of the machines were also recorded.

4. General work conditions included workshop descriptions such as room volume, number of machines, visual condition, room temperature, general ventilation, and ventilation maintenance. Personal protection equipment (PPE) donned by the worker and the use of spray guns were also recorded.

Data treatment

The descriptive statistics of the quantitative measurements (metals in oils, airborne exposure concentrations) were medians and interquartile ranges (IQR). The recorded shift-specific percentages of tasks (Machining, Settings, Machine cleaning, Switching tool, Finishing, Maintenance, Handling, Displacement, Other) were used to generate country-specific work typologies. These typologies were obtained using average complete hierarchical cluster analysis using a L1 distance on the activity percentages. These parameters (complete linkage and L1 distance) were chosen to obtain balanced clusters. The number of clusters in each country were based on the Duda-Hart stopping rule³³.

Log-transformed concentrations of respectively particulate and gaseous phases of personal measurements of workers exposed to MWF were modelled fitting multiple linear mixed-effect models for repeated measurements considering workers as random-effects and the potential determinants of exposure as fixed effects. Therefore, nonexposed workers were not included in these models. In these models all potential determinants were included in increasing stepwise manner adding each variable to the models if it was significant at a 0.05 level. Thus, not all determinants significant in single factor (univariate) models were included. However, the final models included all determinants which were thus selected when modelling either particulate or gaseous fractions.

All analyses were carried out with STATA statistical software, version 16–18 (STATA, College Station, TX, USA).

Ethical approval

This study follows the guidelines of the Helsinki Declaration; all subjects were fully informed about the study’s aims and gave their free and informed consent prior to inclusion. The study taking place in Switzerland was approved by the Swiss Ethics Committees on research involving humans (CER-VD, 2017-00630) and by the Comité de protection des personnes, Sud-Est IV, Lyon, France (reference 2017-A01238-45) for the French part.

Results

Company description and worker activities

Ninety-eight workers (33 controls, 31 exposed to straight MWF and 34 to water-based MWF) were followed over two consecutive days. Table 1 describes the 20 different workshops. Most of the camshaft machines (CAM) were 40–60 years old and had only homemade splash guards and no local exhaust ventilation. On the contrary, automated computer numerical control (CNC) machines were more recent (2004–2022) and equiped with enclosures. Some CNCs were directly connected to a dedicated ventilation system or electrofilters. All companies had general ventilation installed. Companies used from one to four different MWFs (Table 1, Supplementary material Table S1).

In France, workers operated between 1 and 2 machines per shift (maximum 5) were as in Switzerland, this number increased to a median of 5 machines, reaching up to 30 for some CAM operators. French workers tended to move more within the workshop than Swiss workers. The setting activity was more common in Swiss compared to the French workshops.

Organic carbon and metal content of the new and used MWFs

A total of 28 new (16 straight and 12 aqueous) and 44 used (20 straight and 24 aqueous) MWF were collected. The median concentration of OC reached 0.72 (IQR 0.62–0.79) mg OC/mg MWF and 0.74 (IQR 0.67–0.80) mg OC/mg MWF for new and used straight MWF respectively. For water-based MWF, the median concentration was 42.3 (IQR 20.0–242.3) mg OC/mL MWF and 31.7 (IQR 20.7–71.6) mg OC/mL MWF for new and used MWF respectively. The distribution of the six OC fractions degrading at different temperatures in these MWF is illustrated in Fig. 1.

Figure 1.

Distribution of the six OC fractions degrading at different temperature for new and used MWF and for the corresponding aerosol sampled in the workplaces. n corresponds to the number of samples used to calculate the OC distribution.

The percentage of each OC fraction found in new and used MWFs were not statistically different (Supplementary material, Figs. S2, S3). The metal content in new and used MWF is given in Table 2. Chromium and Titanium were not quantified in any of the MWF considered (< 0.2 µg/g). The used MWF were enriched with metals like Cu, Pb and Zn for straight MWF and Al, Ca, Fe, and Zn for water-based MWF.

Table 2.

Median concentrations for metals in new and used MWF based on n samples, with interquartile range in bracket.

	Straight				Aqueous
	n	New [µg/g oil]	n	Used [µg/g oil]	n	New [µg/g oil]	n	Used [µg/g oil]
Al	10	2.4 (< 0.2–4.8)	14	4.4 (0.5–5.5)	11	< 0.2	23	8.2 (1.1–24.7)
B	10	< 0.2 (< 0.2–0.5)	14	0.4 (< 0.2–0.5)	11	1.4 (< 0.2–2.1)	23	1.3 (< 0.2–2.9)
Ca	10	3128 (< 0.2–6405)	14	5205 (62–5962)	11	38 (1–46)	23	110 (53–148)
Cu	10	< 0.2	14	20.8 (5.1–111.7)	11	< 0.2	23	1.2 (0.3–3.2)
Fe	10	0.6 (0.1–1.1)	14	3.1 (1.4–5.0)	11	0.3 (< 0.2–1.1)	23	9.5 (4.6–56.9)
Mn	10	< 0.2	14	0.5 (< 0.2–1.1)	11	< 0.2	23	0.5 (< 0.2–0.7)
Ni	10	< 0.2	14	< 0.2	11	< 0.2	23	< 0.2 (< 0.2–0.6)
Pb	10	< 0.2	14	177 (11–449)	11	< 0.2	23	0.2 (< 0.2–3.2)
W	10	< 0.2	14	< 0.2 (< 0.2–0.9)	11	< 0.2 (< 0.2–0.6)	23	< 0.2 (< 0.2–0.3)
Zn	10	1.0 (0.2–4.9)	14	22.0 (11.8–79.8)	11	0.7 (0.2–2.7)	23	7.9 (4.2–20.6)

These concentrations are expressed as µg/g oil and the limit of quantification for all metals in MWF is 0.2 µg/g oil.

Full shift exposure concentrations

A total of 162 personal and 120 stationary samples were collected for 31 workers exposed to straight oils, 34 exposed to water-based MWF, and 33 workers in the reference group. No personal sampling was collected for 17 office workers assuming that the stationary samples were equivalent.

Table 3 presents the exposure results for personal (particulate and gaseous fraction of MWF aerosol) and stationary samples (OC, metals, aldehydes). Workers using straight MWF were exposed to about five times higher concentrations of gaseous hydrocarbons than workers using water-based MWF. The inhalable MWF particulate fraction was not statistically significantly different between workers using straight oil compared to water-based MWF (Supplementary material Fig. S4). There was no significant difference between personal and stationary measurement regardless of the oils (Supplementary material Fig. S4).

Table 3.

Median exposure concentrations for the personal particulate and gaseous fraction of MWF, and stationary concentrations of OC and metals in the inhalable particulate fraction as well as aldehydes in the gaseous fraction sampled in the different workplaces.

	Non exposeda			Straight MWF			Water based MWF
	Median	Q1	Q3	Median	Q1	Q3	Median	Q1	Q3
MWF	n = 63			n = 66			n = 62
Particulate fraction [µg/m3]	20	[5	37]	193	[102	326]	168	[64	386]
Gaseous fraction [µg/m3]	138	[58	436]	1462	[322	2362]	330	[101	699]
Metals (particles)b	n = 36			n = 52			n = 34
Al [ng/m3]	< 29	[< 35	822]	73	[< 28	566]	133	[< 35	1607]
B [ng/m3]	929	[< 32	5108]	2584	[< 25	10176]	< 34	[< 36	2690]
Ca [ng/m3]	1720	[< 32	4681]	2767	[< 26	5233]	< 32	[< 38	4948]
Fe [ng/m3]	288	[< 31	890]	456	[< 28	1377]	613	[336	3255]
Ni [ng/m3]	121	[< 31	233]	48	[< 35	196]	170	[87	271]
Zn [ng/m3]	< 33	[< 52	114]	< 33	[< 34	1003]	< 33	[< 50	246]
OC (particles)	n = 38			n = 53			n = 34
Conc. OC [µg/m3]	28.0	[19.5	36.0]	180.4	[54.3	266.8]	98.3	[42.2	155.7]
%OC [< 130 °C]	9.2	[6.3	11.3]	8.3	[6.4	10.2]	7.5	[5.4	13.0]
%OC [130 < 250 °C]	30.0	[27.2	32.4]	32.0	[29.4	35.6]	27.4	[25.3	29.2]
%OC [250 < 350 °C]	21.3	[18.9	24.9]	41.1	[31.6	44.9]	29.7	[24.4	33.1]
%OC [350 < 450 °C]	22.1	[19.1	25.9]	12.2	[8.9	19.1]	20.9	[17.6	23.0]
%OC [450 < 650 °C°C]	10.6	[9.6	12.3]	3.7	[2.8	6.3]	8.2	[7.4	10.0]
%OC [> 650 °C°C]	4.8	[3.9	5.9]	2.0	[1.0	3.0]	4.3	[3.1	6.0]
Aldehydes (volatile)	n = 37			n = 52			n = 34
∑15 Aldehydes [µg/m3]	40.9	[29.9	60.2]	32.4	[28.7	65.5]	43.5	[24.7	77.2]
Formaldehyde [µg/m3]	11.2	[5.4	14.3]	5.4	[3.9	7.0]	6.1	[3.5	8.0]

25% (Q1) and 75% (Q3) interquartile range are in brackets.

^aWorkplaces without use of MWF (administrative, quality control, assembly).

^bNon-detected metals (Cu, Mn, Pb, W) in the inhalable particulate fraction are not indicated.

OC compositions of the aerosolized MWF particles were dependent on the oil type (Table 3). The aerosols sampled in workshops using straight MWF were enriched with compounds degrading at an intermediate temperature (fractions 130–250 °C and 250–350 °C) compared to water-based MWF, whose particulate fraction of the aerosol was enriched with OC compounds degrading at higher temperatures (fractions 350–450 °C, 450–650 °C and > 650 °C) (Supplementary material Fig. S5). The metal contents of the aerosolized particles were quite low, regardless of the type of MWF, and are far below their respective occupational exposure limits in France and Switzerland. Cu, Pb and Zn concentrations in used straight MWF and Zn in used water-based MWF were quite high (Table 2), but not detected in the aerosolized particles (Table 3). No statistical correlations were observed between metals in used MWF and metals in the particulate fraction of the aerosol (data not presented). The air concentrations of gaseous formaldehyde and the ∑15 aldehydes were close to or below the limits of quantification (LOQ) in all companies. The median formaldehyde concentration measured in reference workspaces (no-MWF use) was slightly higher than in workshops using MWF.

Determinants of exposure

Three different clusters of activities were defined for workers exposed to water-based MWF and six for workers exposed to straight MWF (Table 4). Most of the workers had two to three tasks. The mixed effect model results are presented separately for the inhalable particle mass (Table 5) and gaseous (Table 6) fractions of the MWF. All models used the same reference situation: machining at room temperature with water-based MWF (or straight MWF in France for the case of straight MWF) in enclosed CNC without hydraulic fluids and using MWF purification system integrated in the machine. Important determinants explaining the exposure to the particulate fraction for all participants were related to their activity, machine enclosure in addition to the workshop temperature (Table 5). Compared to machining activities in France using straight MWF in enclosed CNC, machining activities in Switzerland with straight MWF generated particulate exposure 2.5 times higher whereas setting and maintenance activities decreased this exposure by 2.3–3 times. Absence of machine enclosures strongly increased the particulate exposure during use of straight MWF. Increased workshop temperature decreased the particulate exposure, an effect mainly observed for water-based MWF. For workshops using water-based MWF, handling activities appeared to increase the exposure to particles, although of borderline significance (p = 0.077).

Table 4.

Typical percentage distribution of unit tasks within each cluster for workers using straight or water-based MWF.

	Country	Machining	Settings	Machine cleaning	Switching tools	Pieces finishing	Handling	Machine maintenance	Moving in the workshop	Others
Water-based MWF
F: machining aqueous	France	45.3	6.8	3.4	2.1	2.5	9.6	1.8	20.8	8.0
F: displacement	France	10.1	11.5	0.9	5.5	3.9	13.5	3.5	28.6	22.6
F: handling	France	6.2	2.4	3.6	0.0	31.6	38.6	1.6	11.2	4.7
Straight MWF
F: machining straight	France	41.3	8.4	2.4	1.7	0.8	6.1	5.5	25.0	9.0
S: machining straight	Switzerland	38.3	20.0	1.7	18.3	5.0	5.0	0.0	0.0	11.7
S: machining/setting	Switzerland	40.6	51.1	1.1	2.1	0.0	0.0	2.9	2.2	0.0
S: setting	Switzerland	12.0	76.0	0.9	6.3	0.9	0.0	3.3	0.4	0.3
S: setting/switching tools	Switzerland	2.7	46.8	0.0	29.5	1.4	0.0	8.2	4.5	6.9
S: switching tools/finishing	Switzerland	0.0	0.0	0.0	35.0	32.0	0.0	0.0	0.0	33.0
S: maintenance	Switzerland	0.0	6.7	10.0	2.5	4.2	1.7	71.7	0.0	3.3
Total		22.2	26.3	2.3	6.6	6.5	9.7	6.3	12.3	7.9

Table 5.

Mixed-effect models of the personal exposure to the mass of the MWF particulate phase based on activity clusters and determinants of exposure considering all measurements or for each fluid type.

	All measurements (n = 129)		Straight MWF (n = 67)		Water-based MWF (n = 62)
	Coefficient	p	Coefficient	p	Coefficient	p
Activity cluster		0.002		< 0.001		0.077
F: machining aqueous (n = 28a)	–b				–
F: displacement (n = 17)	0.16				0.13
F: handling (n = 17)	0.40				0.27
F: machining straight (n = 10)	0.09		–
S: machining straight (n = 6)	0.14		0.40
S: machining/setting (n = 9)	− 0.48		0.09
S: setting (n = 23)	0.31		− 0.37
S: setting/switching (n = 11)	0.28		0.14
S: switching/finishing (n = 2)	− 0.37		0.16
S: maintenance (n = 6)	0.23		− 0.48
Machine enclosure		0.019		0.010		0.766
With enclosure (n = 107 = 55 + 52c)	–		–		–
Without enclosure (n = 22 = 12 + 10)	0.40		0.51		0.10
Use of hydraulic fluids		0.321		0.170		0.559
No (n = 64 = 34 + 30)	–		–		–
Yes (n = 65 = 33 + 32)	− 0.10		− 0.19		0.12
Machine type		0.404		0.362		0.489
CNC (n = 70 = 30 + 40)	–		–		–
CAM (n = 31 = 29 + 2)	0.27		0.25		0.25
Grinding wheels (n = 10 = 0 + 10)	0.17				0.56
Other (n = 18 = 8 + 10)	0.14		0.25		0.28
MWF management		0.280		0.125		0.571
Reservoir machine cleaned (n = 87 = 47 + 40)	–		–		–
Not recycled (n = 18 = 10 + 8)	− 0.25				− 0.43
Centralized cleaned (n = 8 = 2 + 6)	0.04				0.04
Not cleaned (n = 16 = 8 + 8)	0.26		0.32		0.19
Workshop temperature		0.005		0.139		0.003
19°–21° (n = 52 = 16 + 36)	–		–		–
22°–24° (n = 36 = 14 + 22)	− 0.50		− 0.57		− 0.63
25°–29° (n = 41 = 37 + 4)	− 0.37		− 0.37		− 0.55
Intercept	2.23		2.00		2.21
Between worker variance	0.09		0.06		0.11
Within worker variance	0.06		0.08		0.04

^aNumber of measurements.

^bReference situation for each determinant.

^cTotal number of measurements = number of measurements used for straight MWF + water-based MWF.

Table 6.

Mixed-effect models of the personal exposure to the mass of the MWF gaseous phase based on activity clusters and determinants of exposure considering all measurements or for each fluid type.

	All measurements (n = 129)		Straight MWF (n = 67)		Water-based MWF (n = 62)
	Coefficient	p	Coefficient	p	Coefficient	p
Activity cluster		< 0.001		< 0.001		0.0820
F: machining aqueous (n = 28a)	–b				–
F: displacement (n = 17)	0.02				0.23
F: handling (n = 17)	0.09				0.26
F: machining straight (n = 10)	− 0.01		–
S: machining straight (n = 6)	− 0.40		0.32
S: machining/setting (n = 9)	− 0.08		0.91
S: setting (n = 23)	0.79		1.46
S: setting/switching (n = 11)	0.63		1.46
S: switching/finishing (n = 2)	− 2.22		− 0.93
S: maintenance (n = 6)	0.39		1.01
Machine enclosure		0.040		0.215		0.666
With enclosure (n = 107 = 55 + 52c)	–		–		–
Without enclosure (n = 22 = 12 + 10)	0.40		0.36		0.11
Use of hydraulic fluids		0.364		0.667		0.025
No (n = 64 = 34 + 30)	–		–		–
Yes (n = 65 = 33 + 32)	0.10		− 0.09		0.36
Machine type		0.078		0.104		0.664
CNC (n = 70 = 30 + 40)	–		–		–
CAM (n = 31 = 29 + 2)	0.45		0.62		− 0.22
Grinding wheels (n = 10 = 0 + 10)	− 0.14				0.31
Other (18 = 8 + 10)	− 0.06		0.08		0.39
MWF management		0.167		0.234		0.029
Reservoir machine cleaned (n = 87 = 47 + 40)	–		–		–
Not recycled (n = 18 = 10 + 8)	− 0.54				− 1.09
Centralized cleaned (n = 8 = 2 + 6)	0.41				0.24
Not cleaned (n = 16 = 8 + 8)	0.14		0.36		− 0.11
Workshop temperature		0.332		0.535		0.003
19°–21° (n = 52 = 16 + 36)	–		–		–
22°–24° (n = 36 = 14 + 22)	− 0.10		− 0.47		− 0.24
25°–29° (n = 41 = 37 + 4)	− 0.27		− 0.33		− 0.54
Intercept	2.42		1.78		2.29
Between worker variance	0.10		0.21		0.00
Within worker variance	0.11		0.04		0.15

^aNumber of measurements.

^bReference situation for each determinant.

^cTotal number of measurements = number of measurements used for straight MWF + water-based MWF.

When considering the gaseous MWF fraction, a smaller number of significant exposure determinants was identified in the models (Table 6). Activity clusters were again of importance for explaining MWF gaseous exposures for all workers. Activities related to machine setting and maintenance were associated with a high mass of gaseous straight MWF, and low for workers associated with switching tool/finishing activities. This model suggested that the CAM emitted more gaseous compounds compared to CNC machines, but the statistical significance was borderline. For water-based MWF, using hydraulic fluids increased the mass of the gaseous fraction while a reduction was observed when water-based MWF were not recycled but completely replaced by new MWF and when temperature of the workshop increased.

Discussion

Our study approach focused on characterizing MWF sources (new and used MWFs in sumps) and identifying important exposure determinants explaining the measured personal particulate and gaseous MWF exposures. This approach would help to better anticipate workers’ exposures to straight and water-based MWF and implement exposure reduction strategies.

One main result of this study was that the oil mists in workshops are not simply aerosolised MWF. Rather, a compositional change between the MWF and the generated aerosols was observed for both OC and metal content. For OC, these changes were dependent on the MWF type. Results presented in Fig. 1 and Fig. S6 in Supplementary Material suggest that machining with straight MWFs generated an aerosol with increased amounts of compounds with intermediate thermal stability compared to used MWFs. A possible explanation is an oxidative degradation of hydrocarbons present in the MWFs²⁰. On the contrary, machining with water-based MWF tended to generate aerosols with lower amounts of volatile components and greater amounts of higher thermal stability components (Supplementary material, Fig. S7). This suggests that water-based MWFs were more prone to pyrolysis at the tool-machined interface with high temperatures (350–750 °C¹³) compared to straight MWFs. Since oil mist is relatively stable and long lasting in indoor air¹⁶, aging processes could also occur and bring additional changes to the aerosolized MWF compositions.

Some metals (particularly Cu, Pb and Zn for straight MWF and Al, Fe and Zn for water-based MWF) tended to concentrate in the sumps during use (Table 2). Clearly, the presence of these metals was the result of machined metal, mainly brass and bronze for straight MWFs, and stainless steel and aluminium for water-based MWF (Table 1). Contrary to the sumps, airborne MWF contained fewer metallic elements and if at all, at very low concentrations (Table 3). The metal distribution in used MWF collected from the sumps were quite different from the air concentrations collected at the same time and were not correlated. This result suggests that metals are mainly quenched in the MWF. Only one study has determined the relationship between metal concentrations in MWF sumps and air concentrations in the workshops³⁴. They reported concentrations of Fe, Mn and Zn in sumps in the range of 1–100 mg/L in workshops machining stainless steel and very low metal air concentrations, which concur with our results. Although 84% of Pb concentrations in the air of the workshops was below LOQ (35 ng/m³), Pb concentrations were very high in straight MWF sumps. In the studied workshops machining non-ferrous metals (e.g., brass), the sumps concentrations for Cu and Pb were between 100 and 200 mg/L. Based on the Basel convention on the Control of Transboundary Movements on Hazardous Wastes and Their Disposal³⁵, used oil with Pb levels > 100 ppm must be considered as hazardous wastes and be treated. Skin exposure to MWF and degreasing solvents were often observed during the sampling campaigns. These solvents might alter the skin barrier and render skin more permeable to metals already known to penetrate skin, such as Cu, Pb, Cr, and Ni³⁶. Pb concentration in our study were similar to a study showing significant skin permeation of Pb (29–132 µg/g) and thus, contribute to the overall Pb blood levels³⁷. This exposure route should be considered in further studies.

This study confirms the importance of measuring not just the particulate, but also the gaseous MWF fraction in air to properly assess workers’ exposures to MWF. The median inhalable particulate MWF mass fraction measured in this study was similar to others recently reported values^9,19,29,38, which were all below 230 µg/m³. Characterisation of the gasous MWF fractions have so far been poorly investigated. Previous published scientific studies have targeted specific compounds such as ethanolamine³⁹ or formaldehyde²⁹, but only Lillienberg¹⁹ and Koller⁹ measured the gasous MWF fraction in parallel with the inhalable particulate MWF fraction. The concentration range was 0.2–29.4 mg/m³ for gasous MWF compounds found in Swedish workshops using straight, water-based, and synthetic MWF and had a mean ratio (gaseous mass)/(particulate + gaseous mass) between 84 and 94%¹⁹. This ratio is similar to our study (88% for workshops using straight MWFs) but higher than the 66% for water-based MWFs in this study. Lower gaseous to total airborne MWF mass ratio for water-based MWFs were observed by Koller et al.⁹ who reported ratios between 40 and 95% in samples collected in Swiss workshops. The presence of a larger mass of gaseous MWF compounds in the air observed for straight MWFs compared to water-based MWFs could result from differences in MWF viscosity. Indeed, evaporation of MWFs from filters was less than 5% for cutting fluids with viscosities greater than 18 Cst (at 40 °C)¹⁵. Materials safety data sheet for all water-based MWF encountered in our study indicated viscosities above this value, whereas most straight MWFs presented viscosities below 18 Cst (Supplementary material Table S1).

Aldehyde concentrations were quite low (< 80 µg/m³, Table 3) and similar between workshops using straight or water-based MWFs. Such concentrations are similar to concentrations found in ambient air⁴⁰ and are comparable with formaldehyde concentrations in workshops using MWF^19,20,41. As the European regulation has prohibited the use of formaldehyde in the formulation of MWF⁴², the presence of this carcinogenic compound could result from the oxidative degradation of MWFs²⁰.

The European standard (EN 689⁴³) describes a strategy for assessing compliance with occupational exposure limits (OEL) for single chemical substances as well as for chemical mixtures. We defined two similar exposure groups (SEG): one for workers exposed to straight MWFs and a second for workers exposed to water-based MWFs. Workers for both SEGs were also exposed to formaldehyde (as a representative of all aldehydes) and metals including boron, calcium, iron, and nickel as predominant elements in the aerosol (Table 3). Based on the EN 689 procedure to compare results with OELs, we calculated an exposure index for each selected pollutant (Tables S2, S3 in Supplementary material). Using this approach, most of the chemical agents contribute less than 0.1 of the OEL, except for the particulate fraction of straight or water-based MWF. We thus tested the compliance of these measurements with the OEL based on the comparison of the 70% upper confidence limit with the 95th percentile of the distribution of the results. The results obtained (Table S4 in the Supplementary material) indicated that there was still a greater than 5% probability of MWF exposures (particulate or particulate + gaseous fraction) exceeding the existing OELs or recommended values in the surveyed workshops. Although aerosol concentrations of formaldehyde, aluminum, and nickel were very low relative to their respective OELs (Tables S2, S3, Supplementary material), they are associated with skin sensitization (for Ni), asthma (for Al and Ni), and cancer (formaldehyde and Ni) where no toxicological threshold exists. Furthermore, possible synergetic or antagonistic effects of such co-exposures cannot be excluded⁴⁴. Therefore, reducing MWF exposures and other pollutants in these workshops is still warranted²⁹.

Process types are considered key determinants for exposure to MWF aerosols³². The processes we followed in this study were always in combination, such as turning combined with drilling and deburing to produce the metal piece, making it challenging to pinpoint process type as a precise exposure determinant. Consequently, we preferred constructing clusters of activities, as country-dependent differences were also observed (Table 4). These clusters were important determinants for explaining personal exposure to the particulate or gaseous fractions (Tables 5, 6).

For particulate exposure when using straight oil, about three times higher MWF concentrations were observed during machining in Switzerland compared to France. Differences in machines type and enclosure (more CAM use in Switzerland compared to processes using stamping press in France, Table 1) could explain these differences. As expected, particulate exposure reduced (2.3–3 times) when machines stopped for setting or maintenance. Considering water-based MWF, handling tasks generated greater (about 2 times) aerosolized particules compared to CNC machining (used as the reference in our models). This could result from covering the machined parts by a thin MWF film for corrosion protection before being transported or using the compressed air gun before packing and shipping the metal pieces.

This study is the first to our knowledge applying the exposure determinant approach to the gaseous fraction of MWF (Table 6). For straight MWF, setting activities including changing tools and machines maintenance generated very high levels of gaseous components and were the only determinants significantly associated with such an exposure. This surge could be associated with the use of compressed air gun to clean the inside of the machine or rapidly opening the CNC’s doors after testing new settings.

For water-based MWF, the task of handling packaged parts was again an important exposure determinant for the gaseous fraction, but of borderline significance. The evaporation of this protective MWF layer could be at the origin of this increased exposure.

The particulate MWF exposure was greater with CAM machines compared to CNC and could be related to the absence of enclosure for the CAM. Indeed, we showed this to be a significant determinant explaining particulate exposure for workers using straight MWF (Table 5). The absence of enclosure induced up to three times more particulate exposure to straight MWF. This value is in line with Lillienberg¹⁹, who report an increase of the inhalable particulate fraction of about 1.5 times for partly open machines. The importance of enclosed systems is also stressed for transfer lines, with a reported reduction of about 90% of the particulate MWF concentration²⁸.

Machine type and enclosure were not significant exposure determinants for the gaseous fraction (Table 6). This illustrates the difficulty to control the emissions of volatile compounds without considering ventilation and elimination of recirculating air²⁹.

The use of hydraulic fluids significantly increased the gaseous fraction of water-based MWF (Table 6). This has previously been reported in laboratory experiments where the presence of tramp oils in water-based MWF increased the misting potential⁴⁵. This could explain the significant contribution of this determinant for the gaseous fraction measured in workshops using water-based MWF.

MWF management was a significant exposure determinant for managing gaseous fraction from water-based MWF. A large reduction of the exposure to the gaseous fraction was achieved when the used MWF was not recycled but eliminated through the fluid purchaser.

Increased workshop temperatures reduced the mass of both particulate and gaseous MWF fractions, especially for water-based MWF. Such temperature exposure effect has been reported previously²³.

This study belongs to a larger and broader Franco-Swiss epidemiological research focussing on occupational exposures to MWF with the aim to assess whether MWF exposure is associated with biomarkers of oxidative stress, genotoxic effects as well as respiratory symptoms³⁰. The consideration of the sumps as sources of contaminants in addition to the joint characterization of the particulate and gaseous fraction should also be noted. Finally, exposure variables like the organic carbon and the metal concentrations are not often reported and bring new insight on the MWF transformations and emissions.

Like all field studies, these results also present limitations. The first one relates to the selection of companies, which was based on a willing to participate rather than to be representative of the present exposures, and thus, our results might not be generalizable to all companies using MWF. A further limitation in the modelling of the determinants is the collinearity between some determinants. In particular, the machine type is highly correlated with the machine enclosure as all CNCs had an enclosure, whereas the grinding wheels mostly hadn’t. This might have led to the absence of the expected effect of enclosure. Some determinants were difficult to collect like the use of compressed air, the machining parameters, or the frequency of MWF changing. These determinants could not be considered in the statistical models. Whereas the recording grid for the exposure determinants was done jointly, differences in recording activities or other determinants by both hygienist teams could be present and thus induce an observational effect. This could have an influence on the activity clusters for example. A stepwise approach has been used for the inclusion of each exposure determinant in the statistical models. When not statistically significant, the added determinant was not considered. This lead us for example to not consider the ventilation parameter, which in other studies has been reported as an important determinant.

Conclusions

Compositional changes are observed between the sump and the particulate fraction of the MWF aerosol. Metals concentrate mainly in the sumps and levels of Pb in this liquid could participate to exposure through skin, particularly when machining Pb-containing alloys with straight MWF. The mass of the gaseous fraction is the most important contributor to the MWF exposure and should be considered in further exposure assessment to MWF. Differences in activities are important determinants modulating the particulate or gaseous exposure. Tasks such as handling metallic pieces originating from water-based processes or machining using straight MWF without proper ventilation showed significant exposure to inhalable MWF particles. Setting and maintenance were associated with a high exposure to the gaseous MWF fraction. The generation of both gaseous and particulate of MWFs would be greatly reduced if compress aid air guns were not used outside of well-ventilated areas.

Author contributions

NH, PW, and JJS conceived the study. RL, JJS, VD, EB, FA and NH organized the field campaign and RL, JJS, VD, EB, FA realized it. Data base management and formal statistical analysis was done by PW. The funding acquisition and the project administration was done by PW. The original draft was written by RL and JJS and reviewed by all the co-authors. All authors agreed on the final version of the manuscript.

Funding

French Agency for Food, Environmental and Occupational Health & Safety (ANSES, Grant EST-2016/1/166).

Data availability

The anonymized datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-024-69677-w.