## 2 Metrological Study of the Regulatory Framework on the Occupational Exposure to Vibrations
### European Directive 2002/44/EC on Exposure of Workers to Vibrations  
European Directive 2002/44/EC [21] on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (vibration) includes further details on how to assess the risk derived from the occupational exposure to vibrations. The directive defines, characterizes, and establishes the requirements for evaluating the exposure to vibrations, and therefore prevents the possible damages derived from the repeated exposition to this agent. Subject to the part of the body that may be exposed to mechanical vibrations, the Royal Decree distinguishes two types of exposure: Hand-Arm vibration (HAV) and Whole-Body Vibration (WBV). For each of these types the royal decree establishes a series of reference values that must be controlled. Specifically, there are two parameters to be monitored (Table 1) [22]:  
* Daily exposure limit value: 5 m/s\({}^{2}\) in the case of HAV exposure and 1.15 m/s\({}^{2}\) in the case of WVB exposure
* Daily exposure action value: 2.5 m/s\({}^{2}\) for HAV exposure and 0.5 m/s\({}^{2}\) for WVB exposure  
The daily exposure value can be calculated using the A(8) parameter, related to the effective values of acceleration and the period of exposure. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Exposure Type** & **Daily Exposure Limit Value (m/s\({}^{2}\))** & **Daily Exposure Action Value (m/s\({}^{2}\))** \\ \hline Hand-Arm vibration (HAV) & 5 & 2.5 \\ Whole-Body Vibration (WBV) & 1.15 & 0.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Reference values European Directive 2002/44/EC (from the work in [22]). The calculation of this parameter depends on the case under consideration: HAV vibrations or WBV body vibrations. The HAV are evaluated through the CEN--EN ISO 5349-1:2001 [23] and CEN--EN ISO 5349-2:2001 standards [24], while the WBV type vibrations are evaluated according to the CEN--EN ISO 2631-1:1997 [25]. Although obtaining those acceleration values by directly measuring them is considered, the directive considers that the acceleration values may be obtained by means of estimation methods; this option is sometimes preferred by manufacturers and suppliers rather than measurement-based methods (Figure 2). Thus, the assessment of the level of exposure to mechanical vibration is not always obtained from direct measurements of the vibrations suffered by the worker during the actual use of the machinery, it depends on the tests applied to the machinery when it was originally manufactured. Although this circumstance is in some measure solve by the implementation of the CEN/TR 15350:2013 (for HAV) [26], yet the operating conditions might vary a lot, and so the level of vibrations. This is the base for the study of the NER investigated within this research, that is, the variation between the estimated and the actual values could overestimate the level of exposure to this risk of the occupational exposure to vibrations. When estimating the acceleration values, European Directive 2002/44/EC states that the assessment of the level of exposure may be carried out on the basis of an estimation obtained by means of the information provided by the manufacturers concerning the level of emission from the work equipment used, as well as the observation of specific work practices or on measurement. However, this information can greatly differ depending on the manufacturer criteria, the essay type used for testing the machine, or the conditions under which those tests were carried out, among other considerations. Consequently, the level of exposure to mechanical vibration firmly depends on the specific equipment and working methods, as well as the values provided by the manufacturers in each case. Unfortunately, the limitless number of different machines and suppliers with different design and manufacturing processes makes it very difficult to compare the results obtained by the different manufacturer.