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New enforcement support for the assessment of air as a legally protected good in the Canton of Zurich

Evaluation of legally protected goods according to the Contaminated Sites Ordinance (AltlV)

Contaminated sites must be evaluated in a differentiated manner. The Contaminated Sites Ordinance (AltlV) specifies that, in addition to the evaluation of the contamination and release potential of pollutants, the exposure and importance of water, soil and air as legally protected goods also have to be determined during the preliminary investigation already. Specifically, the following questions must be answered during these clarifications: “Can the ground pollutants get into the ground water?” or “What is the potential extent of damage due to air pollution?”

Enforcement support specifying the procedure for clarifications regarding water and soil as legally protected goods have long been available at national and cantonal level from the Federal Office for the Environment (BAFU) and the Office for Waste, Water, Energy and Air (AWEL) of the Canton of Zurich. The procedure for investigations regarding air as a legally protected good has thus far not been regulated in detail. As a result, the clarifications for air as a legally protected good were often only incompletely performed. Although the BAFU did publish enforcement support titled “Sampling and analysis of pore air” in 2015, this document only focused on the performance of pore air measurements. It even points out that the sampling and analysis methods mentioned are not suitable for indoor air. This is unsatisfactory, as the indoor air measurements are often decisive for evaluating air as a legally protected good. In many cases, air as a legally protected good is only relevant when the contaminated site is built up. On open land, the dilution of possible gas emissions in the environmental air is often so strong that there is no risk for humans or animals.

New enforcement support by the Canton of Zurich

In June 2019, AWEL published the document “Evaluation of air as a legally protected good acc. to Article 11 of the Contaminated Sites Ordinance (AltlV), enforcement support for the Canton of Zurich”. This document describes the process regarding air as a legally protected good during the preliminary investigation and the monitoring of polluted sites in detail.

Article 11 of the AltlV states that clarifications regarding air as a legally protected good are required when there is a high probability that both of the following criteria apply:

  • The release (emission) of gaseous substances from the site is possible
  • The spread (migration) of gaseous substances to an immission place (inhabited indoor space) is possible.

The conclusion as to whether a site requires monitoring or remediation is based on indoor air measurements at the place of immission. If gases emitted from a contaminated site can be detected in inhabited indoor environments in a concentration that is above the background contamination, the site requires remediation. If no gases are detected at the site of immission, but migration is possible in principle, monitoring is required.

The enforcement support of AWEL further includes detailed information regarding suitable investigation methods and measuring conditions. It specifies, for example, that indoor air measurements may only be performed when the air pressure is stable. In addition, at least two measurements at an interval of approx. six months are required.

The AWEL recommends a two-step procedure. In a first step, the pollutant concentrations in the indoor air are determined in the areas of possible entry paths (joints, cracks, shafts, etc.) using hand-held measuring devices (display of cumulative parameters). If required, this is followed by the analysis of individual substances for accurate determination of the indoor air pollution. A specialised centre certified by the Swiss Certification Office for Building Products (S-Cert AG) must in this case be involved.

Practical experience

Ecosens AG is an expert bureau certified by S-Cert AG and has performed detailed investigations regarding air as a legally protected good at two contaminated sites during the last year. The respective procedures selected complied with the specifications in the new enforcement support. Both sites are previous industrial sites with ground contaminated with chlorinated hydrocarbons (CHC). In both cases, the previous investigation results did not exclude the possibility that the indoor air in the buildings on that site were polluted with CHC.

In order to investigate this suspicion, the buildings were examined for external sources and entry paths using a photo-ionisation detector (directly displaying, hand-held device). Thereafter, the CHC contamination in the various rooms was determined by indoor air measurements using single substance analysis. Based on these measurements, a toxicological evaluation of the indoor air was performed and a classification regarding air as a legally protected good was issued.

The measurements showed that the detection limits of hand-held measuring devices (e.g. PID) are insufficient for determining the CHC concentration in indoor air. Due to the high toxicity of CHC, the health-based guideline values for internal air are lower than the detection limits of these devices. Furthermore, the devices usually only show the total of all chemicals in the air and are therefore sensitive to interference such as the use of solvents in production facilities. CHC and similar substances therefore require conventional indoor air measurements to evaluate the indoor air.

A further finding of these investigations is that very volatile compounds such as CHC can also penetrate into the building through visually intact floor plates. In both cases, some indoor spaces showed CHC pollution that was significantly above the background contamination. However, based on the results of the indoor air measurements, health risks for the building users could be excluded


Your contact person:

Pascal Diefenbacher
Indoor Air Quality Divisional Manager