Risk factors of renal dysfunction and their interaction in level-low lead exposure paint workers

Chronic lead poisoning may cause malfunction of a variety of organ systems. Since Lanceraux E [1] reported the nephrotoxic effects of lead in 1881, a great deal of researches have been done on renal dysfunction caused by exposure to high level of lead [2, 3]. With further understanding of the lead, government has tightened the supervision of enterprises and enterprises also have raised the awareness of keeping the lead level in workplace within limit. Therefore, the level of lead in the related occupational environments has been reduced by a large margin. Recently, a large number of researches have been devoted to studying the damage of low level lead exposure to the kidney. However, results are inconsistent. Some found that low level lead exposure would result in nephrotoxicity [4, 5], while some large population studies showed that low level blood lead causes no harm to the kidney in general people [6]. It’s worth noting that in most of those studies, serum creatinine has been used as index of renal dysfunction and this might be a limitation since the level of serum creatinine is easily influenced by some other factors like examinees’ diet and metabolism, which could become abnormal at the terminal stage. In addition, most of these studies only focused on differences of serum creatinine between renal dysfunction and renal normal group but neglected other possible effects such as the interaction between blood lead and other factors.

Taking urine β2- microglobulin as index of renal dysfunction and taking workers in either paint using or painting producing enterprises with low level lead exposure as study populations, this paper studied the association between low level of lead exposure and the renal dysfunction and investigated the interactions between low level lead exposure and other factors. The findings could serve as the reference evidence for the formulation of relevant laws and regulations on prevention and control the harm of low level lead exposure.

A total of 780 workers in the two chosen companies met the standard of exposing to paint for at least one year and 752 of them participated in this investigation, yield a 96.41% response rate. Of the 752 participants, 747 has valid result for urine β2-MG, making the valid survey rate as high as 98.13%. Of the 747 workers, 348 were from Sany Heavy Industry and 399 were from Xiangjiang Kansai Paint Company. The participants’ age ranged from 18 to 59 years old (mean age 31.66 ± 7.74 years old). The study population was made up of 706 males and 41 females. Of the study population, 70 workers were blood lead positive, accounting for 9.37% in total participants. The blood lead level in blood lead positive workers was between 0.04 and 0.35 mg/l (mean 0.09 ± 0.06 mg/l), which was below the limit of blood lead level stipulated by Chinese government for the relevant occupations (of 0.40 mg/l). Urine β2- MG level was between 0.02 and 0.69 mg/l (mean 0.20 ± 0.10 mg/l) in all subjects. 93 workers were diagnosed as renal dysfunction, which yield the renal dysfunction rate as 12.37%.

The results of univariate analysis showed that renal dysfunction prevalence rate were higher among participants with longer work year, hypertension and positive for blood lead. There was no statistically significant association between gender, age, smoking, drinking, whether wear mask or not and renal dysfunction (Table 2).

Because univariate analysis found the difference of prevalence between high-normal blood pressure group and low-normal blood pressure group was nonsignificant, those two group was merged into normal pressure group in multi-variable unconditional logistic regression analysis, all variables involved in logistic regression analysis were listed in Table 2, if P value ≤ 0.05, we defined it as positive, otherwise negative. The results of logistic regression analysis showed that renal dysfunction of paint workers was associated with longer working year (OR = 1.699, 95% CI: 1.226~ 2.355), blood lead positive (OR = 2.847, 95% CI: 1.577~ 5.139) and hypertension (OR = 2.192, 95% CI: 1.103~ 4.359) (Table 3).

Of the major risk factors associated with renal dysfunction of paint workers, blood pressure and blood lead were controllable variables, so we further investigated the interactions between blood pressure and blood lead using stratified analysis, the results indicated that when blood pressure was normal, the association between low- level of lead exposure and renal dysfunction was statistically significant (OR = 2.784, 95% CI: 1.475~ 5.225). When blood lead was negative, the association between hypertension and renal dysfunction was statistically significant (OR = 2.181, 95% CI: 1.005~ 4.730). In cases where the hypertension and blood lead positive existed at the same time, the risk of kidney dysfunction was the largest (OR = 8.723, 95% CI: 2.131~ 35.710). RERI, API, and S were 4.758, 54.5% and 2.604 respectively, which indicated that there was a positive interaction between above two factors (Table 4).

Urea nitrogen and creatinine are often used as indicators for renal function in clinical. However, these two indicators only become abnormal at the terminal stage of disease and are often subject to the diet and metabolism of the individuals, thus incapable of reflecting the renal dysfunction promptly and accurately. β2- MG is an alternative indicator for renal dysfunction. It is a small molecular globulin produced by lymphocyte, blood cells and polymorphonuclear leukocytes with a very constant synthetic ratio and burst size. 99.9% of β2- MG filtered by glomerulus could be absorbed by proximal renal tubules. Once the proximal renal tubules are lightly damaged, the amount of β2-MG in urine will soar [7]. Therefore, the amount of β2-MG can acutely reflect the damage of proximal renal tubules. Hence, it is of great significance in helping diagnosing renal dysfunction at its early stage [8]. According to our investigation, 12.37% of the study population suffered from renal dysfunction, which is much higher than some other participants. The investigation of Zhang LX [9] et al. showed that 10.8% of the study population of a national representative sample of 47,204 Chinese suffered from chronic renal dysfunction in 2009. And the prevalence of renal dysfunction of general population in UK and Canada were 5.85% [10] during 2007 to 2009 and 4.54% [11] in 2010, respectively. One hypothesis for the high percentage of renal dysfunction found in our study is the low level of lead exposure has increased the risk of renal dysfunction.

This investigation found that the longer painter work, the greater of the risk they have renal dysfunction. Working year, on one hand, had a positive correlation with age, which itself was a independent risk factor for renal dysfunction [12‐14]; On the other hand, the longer one work, the longer one exposed to the lead. As a result, more leads accumulated in bodies would lead to high risk of renal dysfunction [15].

Our findings showed that hypertension is an independent risk of renal dysfunction. Studies of Bao YS [16] et al. and Bidani AK [17] et al. have shown similar results. One explanation was that changes of the hemodynamics in the kidney of the workers with hypertension lead to disturbance of the vascular activator such as reactive oxygen species and interleukin, which would cause the renal arterioles wall thickening and luminal stenosis, impose high pressure on glomerulus and increse filtering. Consequently, the amount of urine β2-MG go up accordingly [17].

In this investigation, 70 cases were detected positive for blood lead, ranging from 0.04 to 0.35 mg/l (mean 0.09 ± 0.06 mg/l), which are much lower than the limit of 0.40 mg/l stipulated by China and American etc. government for the occupations exposed to lead. Although the blood lead was at a very low level, those who tested blood lead positive were more likely to suffer from renal dysfunction than those tested negative.Some pertinent literature from other countries also found that low-level blood lead might lead to renal dysfunction [6, 15]. Muntner P [6] et al. found, from the NHANESIII (National Health And Nutrition Examination Survey III), that the prevalence of CKD was closely associated with the blood lead level in hypertension patients, with the blood lead level between 0.039~ 0.059 mg/l, the prevalence of CKD is 10.8%, and between 0.025~ 0.038 mg/l is 10.4%. The risk of CKD of these two groups are 1.69 and 1.56 times of those with blood lead <0.024 mg/l, respectively. Although CKD was not completely the same as renal dysfunction, both are associated with impaired renal function. Our study found that blood lead ≥ 0.04 mg/l and hypertension could increase the risk of renal dysfunction independently; moreover, the two factors also had obvious interaction on the renal dysfunction. When blood pressure is normal, the risk of renal dysfunction of those with blood lead positive (Pb-B ≥ 0.04 mg/l) is 2.784 times of those with blood lead negative (Pb-B < 0.04 mg/l). When blood lead level is negative, the risk of renal dysfunction of people with hypertension is 2.181 times of those with normal blood pressure. In cases the workers have both blood lead positive and hypertension, the risk of renal dysfunction was greatest (OR = 8.723), 54.5% of the results could be explained by the interaction between hypertension and blood lead. The possible mechanism of the interaction may included that: ① continuous hypertension causes glomeruli high pressure, and makes the glomerular filtration rate and urine β2- microglobulin increase; ② low level blood lead alone does damage to renal tubular epithelial cells, restrains ATP activity, and leads to tubular reabsorption dysfunction [18]. The co-existence of the two factors will promote renal dysfunction.

Two significant findings are achieved in this paper. One is that blood lead level much lower than the limit of 0.40 mg/l for the occupations exposed to lead may still increase the risk of renal dysfunction. Another is that there is a positive interaction to renal dysfunction between exposure to low level lead exposure and hypertension. From these findings, we can know that the limit for the occupational exposure to lead in China and other countries with same limit is not safety. It is also concluded that when utmost efforts are made to reduce occupational lead exposure, it is of great significance to control blood pressure of occupational population within a normal level to prevent renal dysfunction.

There is no denying that this research has some limitations. First, this is a cross- sectional survey, the lack of prospective follow-up limits the ability to make inferences regaring the causality of the associations. Second, although the utmost efforts were made to explore the influence factors of renal dysfunction, the factors which we analyzed still could not be ensured to be complete, there may exist other factors having something to do with renal dysfunction. Third, new reliable methods like ICP-MS available nowdays, but due to technical limitations, we used Graphite furnace atomic absorption spetrophotometry (WS/T20–1996) for the detection of lead, the accuracy maybe reduced. Finally, due to technical limitations, we didn’t measure other metals that might be associated with renal dysfunction such as Cd and Hg.

Low-level lead exposure and hypertension not only have independent effect on renal dysfunction in paint workers, but also had obvious positive interaction in paint workers. Interventions aimed at blood lead and blood pressure at the same time will better prenvent from renal dysfunction.