Tag Archive | VOCs

Removing gas pollutants; let’s talk Tech!

Three main technologies are used to remove gaseous pollutants. They have different operating principles, energy requirements, potential side effects and efficiencies. Below is a basic description of these technologies.

Photo catalytic oxidation (PCO)

How does it work?

PCO technology uses ultraviolet light (UV) on a catalytic surface (often titanium oxide TiO2). It triggers the formation of  highly reactive species (hydroxyl radicals, ions, ozone,etc. ), which enable VOCs to be decomposed into CO2 and water.

Are there potential side effects?

UVPCO generates ozone, which is harmful in itself (increasing the risk of respiratory diseases) and may also react with other organic compounds to form harmful by-products such as formaldehyde. Alcohols can also create poisoning of the catalyst and cause incomplete oxidation leading to production of aldehydes or unwanted species.


How does it work?

Solid sorbents are materials with large internal surface, enabling electrostatic interaction between the gas molecules and a surface. Some examples include activated charcoal, silica gel, activated alumina, lithium chloride, zeolites to porous clay minerals. Gaseous pollutants removed by physical sorbents include ozone, nitrogen dioxide or VOCs (except low molecular weight carbon compounds such as formaldehyde and ammonia).

For chemisorption gas filters, the working principle is similar but in addition, a bond-forming chemical reaction between the adsorbed gas pollutant molecule and the adsorbing surface occurs, involving electron transfer. One common chemisorbent is potassium permanganate, an active oxidating reagent. It  can convert formaldehyde into water and carbon dioxide. The efficiency of the sorbent, measured via the rate of adsorption, decreases with the amount of pollutants captured. Similarly to mechanical air filters, sorbents require energy to pull the air through.

Are there potential side effects?

Carbon filters have high biocompatibility and microorganisms may multiply on filters.

Plasma cluster ions 

How does it work?

One type of ion generator is called cluster generators or plasma generator (the term DBD or Dielectric Barrier Discharge may also be used). It uses AC discharges to produce clouds of positive and negative ions successively. Electrons are separated from oxygen molecules and the electrons are combined with the oxygen molecules to produce oxygen cations and anions. The oxygen anions react with water in the air and produce reactive species, which then agglomerate and form ion clusters. Ion clusters can damage the surface of airborne microbial and oxidize gaseous volatile organic compounds and PM.

Are there potential side effects?

Ion generators may generate ozone and undesired by-products.  Plasma systems tend to be very popular and displace UVPCO systems although they use a lot more energy.


Type of air pollutants

Air pollutants can basically be divided in two categories:

  1. particles
  2. gas pollutants.

What are Particles? 

Many people are concerned by inanimate Particulate Matter (often called PM), but most of us are unclear of what PM actually is. PM is in fact a mixture of small solid and liquid droplets from acids (such as nitrates and sulphates), organic chemicals, metals, soil and dust particles. PM can be both inanimate and animate (such as microbiological pollutants).

Where do they come from?

Inanimate particles come from anthropogenic sources (like construction sites, agricultural activities, manufacturing, landfills, wood-burning stoves, fireplaces, vehicles, etc.) or natural sources (sandstorms, windblown dust, volcanic activity, forest fires and biogenic sources, e.g. pollen and plant wax).

What is the difference between PM10 and PM2.5?

PM10 is composed of particles of diameter less than 10 micrometers, while particles in PM2.5 have a diameter inferior to 2.5 micrometers. It is important to differentiate between PM2.5 and PM10 because the health impact is different. In particular, the particles PM10 can reach the upper part of the airways and lung, while smaller particles PM2.5 can penetrate more deeply into the lungs and alveolar region.

What about smaller particles ?

There is also a growing concern about ultrafine and nano-particles. They could in fact have more severe effects than larger particles (PM10 and PM2.5). Health impacts of particles listed by EPA (Environmental Protection Agency) include premature death in people with heart or lung diseases, aggravated asthma, decreased lung function and increased respiratory symptoms, such as airways irritation.

What kind of gas pollutants should be tracked?

WHO (World Health Organization) published air quality Guidelines in 2005. It includes a list and to description of major gas pollutants creating a risk for health. Major gas pollutants (excluding VOCs), are:

  • Carbon Monoxide (CO), interferes with oxygen-carrying capacity of blood.
  • Nitrogen dioxide (NO2), irritates the lungs, lower resistance to respiratory infections and increases the risk of miscarriage. NO2 is key precursor of nitrate particles forming important share of PM2.5.
  • Sulphur dioxide (SO2) dissolves in water vapour and form sulphuric acid. Interacting with other gases, it is then forming sulphates. Sulphate particles are one element of PM2.5.
  • Ozone (O3) formed naturally & necessary to protect from UV radiation in the stratosphere. But ground level ozone (formed from chemical reaction of oxides of nitrogen and volatile organic coumpounds (VOCs) in the presence of light and heat) can irritate the airways of the lungs & increase asthma symptoms, and cardiopulmonary problems.

Common sources of air pollution gas SO2, NO2 or CO2 are fossil fuels combustion (road transport, shipping, electricity generation, heating plants, industrial processes, cooling and gas stoves) and refining processes of coal, oil, metal-containing ores (SO2, CO2). Main sources of O3 include printers, copiers, UV bulbs, ozone generators.

What are VOCs? Why should we care?

Volatile Organic Compounds (commonly called VOCs) are more and more discussed amoung experts. They are chemical compounds containing one or more carbon atoms, which can evaporate at room temperature and normal atmospheric pressure. More than 900 different VOCs were identified by the U.S. Environmental Protection Agency in indoor environment (1989) at concentrations higher than 1 ppbv (concentration units of parts per billion by volume). The complexity to study VOCs’ impact on health comes from the fact that VOCs are harmful but typical not acutely toxic. Instead, they can have long-term and chronic health effect at low concentration. These pollutants are mainly emitted by indoor sources (building materials, cleaning agents, cosmetics, waxes, carpets, furnishings, laser printers, photocopiers, adhesives, paints, etc.). Concern about VOCs is growing but more research needs to be done on targeted VOCs. VOCs include benzene, formaldehyde, toluene, limonene, xylene, etc.

What are SVOCs?

Recently, more research has been focused on Semi Volatile Organic Compounds (SVOCs), which are particularly difficult to handle with ventilation. They are used in many products (e.g. flame retardants) and may have hormones disrupting effect. For more information on Endocrine Disrupting Chemicals, I recommend reading the WHO report of 2012.