Morphological characterization
The sample collection and analysis procedure described in the previous section resulted in the detection of actinolite, as indicated in Table 1. The morphological characteristics of representative particle types observed in the park landscape rock samples are compared with those of the standard asbestos sample in Fig. 1. Here, most of the park landscape rock samples display thick and uneven particles, with a completely distinct morphology from that of the standard asbestos sample with its straight, thin, long particles. In addition, the ends of the park landscape rock particles are seen to be uneven and irregular in shape due to splitting, while the HSE standard samples exhibit predominantly straight, square-ended particles.
Table 1
Characteristics of bulk sample from the landscape rock
Sample
|
Number of
bulk sample
|
Number of
Asbestos-containing bulk sample
|
Asbestos type
by TEM
|
1
|
4
|
2
|
Actinolite
|
2
|
6
|
3
|
Actinolite
|
3
|
10
|
4
|
Actinolite
|
4
|
4
|
3
|
Actinolite
|
5
|
5
|
3
|
Actinolite
|
total
|
29
|
15
|
Actinolite
|
Particle Width Distribution And Length-to-width Ratio Distribution
The particle-width distributions of the asbestos fibers detected in the landscape rock are compared with those of the standard asbestos samples in Table 2, and the corresponding length-to-width ratio distributions are compared in Table 3. In the case of the landscape rock samples, the average width of the fibers is seen to range from 1.5 to 2.1 µm, with 2–7% of the fibers having widths of 0.5 µm or less, and 11–31% having widths of 1.0 µm or less. Meanwhile, the average length-to-width ratio of the fibers is 6.1–8.2, with 7–21% of the particles having length-to-width ratios of 10:1 or more, and 0–5% having length-to-width ratios of 20:1 or more. In the case of the HSE standard asbestos samples, the average particle width is 0.3 µm, with 98% of the fibers having widths of 0.5 µm or less, and all of the particles having widths of 1.0 µm or less. Further, the average length-to-width ratio of the standard asbestos particles is 67.1, with 99% of the fibers having length-to-width ratios of 10:1 or more, and 96% having length-to-width ratios of 20:1 or more.
Table 2
Width data of actinolite 5 ㎛ and longer with a minimum aspect ratio of 3:1
Sample
|
Particles
analyzed
|
Mean
(㎛)
|
Width ≤ 0.5 ㎛
(%)
|
Width ≤ 1.0 ㎛
(%)
|
1
|
100
|
2.1
|
2
|
11
|
2
|
100
|
1.9
|
6
|
22
|
3
|
100
|
1.5
|
7
|
31
|
4
|
100
|
2.1
|
2
|
13
|
5
|
100
|
1.8
|
2
|
13
|
HSE Reference Actinolite
|
100
|
0.3
|
98
|
100
|
Table 3
Aspect ratio data of actinolite 5 ㎛ and longer with a minimum aspect ratio of 3:1
Sample
|
Particles
analyzed
|
Mean
(㎛)
|
Aspect ratio ≥ 10:1
(%)
|
Aspect ratio ≥ 20:1
(%)
|
1
|
100
|
6.1
|
7
|
0
|
2
|
100
|
8.0
|
9
|
5
|
3
|
100
|
8.2
|
21
|
5
|
4
|
100
|
6.8
|
14
|
2
|
5
|
100
|
6.4
|
10
|
1
|
HSE Reference Actinolite
|
100
|
67.1
|
99
|
96
|
The average width of the five landscaping rock samples is 1.9 µm, which is almost twice the 1 µm width that Wylie et al. associated with human health hazards such as cancer or mesothelioma, based on the results of epidemiological studies and animal experiments [6]. Further, their average length-to-width ratio is 7.1, with 95–100% of particles failing to meet the solid asbestos analysis criteria of 20:1, and no separate asbestiform characteristics were identified. Thus, these samples are considered to be non-asbestiform, according to the definition in the solid sample asbestos analysis method of the U.S. EPA [4]. Moreover, only 0–5% of these particles satisfy the asbestiform criteria proposed by Chatfield [13], having widths of 1.5 µm or less and length-to-width ratios of 20:1 or more. Hence, they are mostly classified as non-asbestiform.
The width and length-to-width ratio distributions of fibrous particles observed in the standard asbestos sample and the landscape rock are further compared in Fig. 2, indicating average widths (± the standard deviation) of 0.3 ± 0.2 µm and 1.9 ± 1.1 µm, respectively. Thus, the width of the standard asbestos samples is much lower than that of the landscape rock samples, with a more uniform distribution. Further, the average length-to-width ratios (± the standard deviation) for the standard asbestos and the landscape rock samples are 67.1 ± 44.8 and 7.1 ± 5.9, respectively. Thus, the standard asbestos samples exhibit an almost constant particle width regardless of the length, leading to a large variation in the length-to-width ratio, whereas the particle width of the landscape rock samples tends to increase as the length increases, to give a more uniform and smaller length-to-width ratio than that of the standard asbestos sample.
Length Versus Width Correlation
The regression equations and correlation coefficients for the log width (y) versus log length (x) of the landscape rock and the HSE standard asbestos samples are presented in Table 4. According to the classification scheme for the shapes of asbestos particles described by Siegrist and Wylie (1980), the most obvious difference between asbestos fibers and non-asbestiform mineral particles is that the width of asbestos fibers is relatively constant regardless of the length, whereas that of non-asbestiform mineral particles varies as a function of length[18]. In the present work, the length and width of the standard asbestos samples show a low correlation, with a correlation coefficient of 0.297, whereas those of the landscaping rocks exhibit a relatively high correlation, with a correlation coefficient ranging from 0.530 to 0.786. Based on these results, the landscaping rock samples show the characteristics of non-asbestiform mineral particles.
Table 4
Regression equation, correlation coefficient of log width vs. log length
Sample
|
Regression equation
Log width = f (Log length)
|
Correlation coefficient
|
1
|
Y = 0.8925X – 0.6437
|
0.786
|
2
|
Y = 0.7521X − 0.5517
|
0.643
|
3
|
Y = 0.8596X – 0.7232
|
0.627
|
4
|
Y = 0.7564X – 0.5384
|
0.691
|
5
|
Y = 0.7055X – 0.4801
|
0.530
|
HSE Reference Actinolite
|
Y = 0.2456X – 0.8328
|
0.297
|