Determination of crystalline silica in respirable dust upon occupational exposure for Egyptian workers

ABSTRACT

Crystalline free silica is considered as a lung carcinogen and the occupational exposure to its dust is a health hazard to workers employed in industries that involve ores of mineral dust. In Egypt, thousands of people work under conditions of silica dust exposure exceeding the occupational exposure limit, as a result the monitoring of this occupational exposure to crystalline silica dust is required by government legislation. The assessment of the later is a multi-phase process, depend on workplace measurements, quantitative analyses of samples, and comparison of results with the permissible limits. This study aims to investigate occupational exposure to crystalline silica dust at 22 factories in Egypt with different industrial activities like stone cutting, glass making, ceramic, and sand blasting. Dust samples were collected from work sites at the breathing zone using a personal sampling pump and a size-selective cyclone and analyzed using FTIR. The sampling period was 60–120 min. The results show that the exposure at each of the industrial sectors is very much higher than the current national and international limits, and that lead to a great risk of lung cancer and mortality to workers.

MATERIALS AND METHODS

In this study, we measured personal exposure of 47 randomly selected workers from ten industrial sectors in 22 Egyptian factories. These samples were collected using portable dust samplers. Each sampler consists of a cyclone (Higgins-Dewell, SKC) loaded with 25 mm cellulose membrane filters, pore size 0.8 μm (SKC) where the respirable dust precipitated and a pump (Gilian GilAir5) which is calibrated with Dry Cal Defender 530 calibrators (Bios International, Butler Park, N.J.). The flow rates were 2.2 l/min and sampling periods were about 60–120 min. The cyclones were fixed on the worker’s clothes attached to the belt and collars at the breathing zone (PZ). Filters were weighed before and after sampling using an electronic balance (model HA-202M, A&D Co., Ltd., Japan) to give the weight of dust collected. The volume of air was calculated from the sampling flow rate and the sampling time. The dust concentration was then derived as equation 1:

Dust Concentration (mg/m3) = Weight of dust collected (mg) / Volume of air sampled (m3)

During sampling a minimum of three unused loaded cassettes with weighed filters were kept as a blank and treated as far as possible in the same manner as those actually used for sampling but without drawing air through them. The method used for the determination of respirable dust is MDHS14/3. This MDHS aims to guide those who wish to measure the concentrations of respirable and/or inhalable dust in air, for the purpose of monitoring workplace exposure. It updates and replaces MDHS 14/2.

In this study, infrared spectra were acquired by means of a Nicolet is 10 FTIR (Thermo scientific, USA) data resolution of 4 cm−1. The number of scans was set at 16–64. Filters loaded with samples for investigation has been ashed in a muffle furnace at 600οC for 2 h. It was then homogenized with 300 mg dried spectroscopic grade potassium bromide (KBr) in an agate mortar. The later was pressed into 3 mm diameter pellets with a hand press.

α-quartz as a standard reference material; from national institute of standards and technology NIST-SRM 1878, was used to prepare the crystalline silica calibration curve. Although crystalline silica has several polymorphs such as cristobalite and tridymite, it is reasonable to use quartz as the standard because the other forms of silica are usually not present in a significant amount in industrial hygiene samples.

The absorbance at 800 cm−1, which is due to the symmetrical stretching vibration of Si-O-Si, was used to find the weight of quartz, Wq (μg), from the calibration graph. The concentration of silica, C (mg/m3) was calculated using equation 2:

Silica concentration= Wq/V mg/m3 (Where V is the volume of air sampled in liter (2)

To calculate the percent of quartz, % Q, we used the equation 3:

% silica = Wq/Ws×100 (Where Ws is the total sample weight) (3)

In decree No. 211/2003 (Egyptian labour law 12/2003), there is only a TLV for respirable dust with free silica content <1% (3 mg/m3) where the TLV for respirable dust containing silica in general industry is determined by the equation 4:

TLV(mg/m3)=10/(%Respirable SiO2+2) (4)

For comparisons to the law, a TLV is calculated for each sample as an 8-h TWA.

This is also the case to calculate TWA concentration for the OSHA PEL as an 8hr TWA, but now OSHA has dropped the TLV calculations and has moved like NIOSH and ACGIH to use just the measured concentration of silica as a limit value.

Dust samples measurement and free silica analysis were made in the national institute for occupational safety and health, Heliopolis, Cairo, Egypt. All samples were analyzed according to MDHS 14/3; a general method for sampling and gravimetric analysis of respirable and inhalable dust1 and NIOSH manual of analytical methods (NMAM), method 7602 (Silica, Crystalline by IR).

RESULTS

In order to obtain calibration curves for crystalline silica, quartz samples of respirable range were used as the standard. The absorption at 800 cm−1 is generally accepted as the most suitable band for analytical peak measurement. Therefore, an ordinary calibration curve is expressed as the graph of absorbance at 800 cm−1 vs. mass of quartz in a sample pellet, Fig. 1.

In this study, we monitored the levels of respirable dust and free crystalline silica in the work environment of 22 factories categorized into 10 industrial activities in and near Cairo industrial zones. 47 samples were collected from these places with mean sampling time of 60–120 min at the personal breathing zone of the workers during a normal work shift.

Table 1 lists the detailed job titles, number of samples for each job title, respirable dust concentration in mg/m3, and the statistical calculations for industries with a number of samples greater than 6.

Considerate to Egyptian standard for respirable dust concentration (3 mg/m3) and regarding to Fig. 2, the respirable dust concentration mean exceeds the permissible limit in all of the industrial sectors except at fertilizers industry during mixing the components of the fertilizers mixture.

The crystalline silica percent with the statistical calculations are listed in Table 2. It was found that silica% exceeds the 50% in the sectors of glass making (during discharging of the sand in the mixture), ceramic (at the worker responsible for the mills), and stone cutting (at drilling machine worker). Figure 3 present the percentage mean of silica with a max mean silica% found in stone cutting at 48.65% and the min mean silica% in painting at 2%.

Table 3 lists the calculated TLV as 8-h TWA for respirable dust concentration containing >1.0% silica in mg/m3 and the statistical calculations for each of the job titles. Figure 4 shows the silica concentration mean with the maximum silica concentration mean at painting industry (2.5) as a result of a minimum silica percent. The minimum silica concentrations mean was at stone cutting industry (0.2) as a result of maximum silica percent.
