The carcinogenic formaldehyde is a ubiquitous indoor pollutant because of its numerous emission sources (cigarette smoke, plywood, isolation foam, adhesive resins, cosmetic, detergents, etc.); its concentrations in homes can vary from a few ppb to more than 100 ppb. Most humans spend 80 to 85 % of their time indoors; accordingly, the Agency for Toxic Substance and Disease Registry (ATSDR) recommends, for a “chronic” exposure, i.e. during an entire lifetime, a formaldehyde concentration as low as 10 μg/m3 or 8 ppb.
A particular challenge in formaldehyde detection is to detect it selectively among other aldehydes also present in the atmosphere. To this end, Fluoral-P was chosen as probe molecule; it and formaldehyde, which both absorb in the UV, react together in solution to produce the cyclic 3,5-diacetyl-1,4-dihydrolutidine (DDL) compound. The selectivity for formaldehyde arises from the fact that only this aldehyde can produce a cyclic compound which absorbs in the visible; a further attractive feature is that the cyclic DDL fluoresces, which greatly assists the detection at sub-ppb levels of formaldehyde when it is required.
The sensor for formaldehyde in the air is a transparent nanoporous monolithic matrix doped with Fluoral-P, which reacts selectively with formaldehyde to produce a yellow product. The intensity of coloration, which is visible to the naked eye, is proportional to the formaldehyde content in the air. In order to measure the concentration precisely, the sensor is placed in a fluidic chamber through which is passed a constant flux of the air to be analyzed. A 20 min exposure time is sufficient to measure formaldehyde concentrations from 5 ppb to 1 ppm; for lower concentrations such as 1 ppb, 60 min is needed. In all cases, the measurement of the yellow color at 410 nm before and after exposure of the sensor to the air gives directly the formaldehyde concentration.
The apparatus commercialized by Ethera, Profil’Air, provides the exposure and reader systems to be used with the sensor cartridges (See : www.ethera-labs.com).