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Air Filtration at Molecular Level


Air filtration, as is commonly understood, is often what we read in advertisements of air-conditioners or what we read in newspapers and journals about the poor quality of air around us, which is mainly filtration of particulate contaminants.

This article, however, will discuss various aspects of air filtration at a molecular level, i.e. filtration/removal of gaseous contaminants. Air filtration at a molecular level is often also referred to as Gas Phase Filtration.

Today, when technologies and equipment, especially in the mission critical facilities require a clean environment to function at their optimum, molecular phase filtration and its understanding is essential.

This article will also discuss the concept, need, technology trends and equipment for molecular phase filtration.

First, let us understand the basics :

Molecular Phase Filtration – Filtration of gaseous contamination having size at the molecular scale.

Particulate Air Filtration – Filtration process that removes solid particulates from the air.

US ASHRAE standard 52.2 classifies particulate filtration as

  • Pre filtration (G class) from MERV 1 to MERV 8
  • Medium filtration (F class) from MERV 9 to MERV 16

The above classification caters more to various domestic, commercial and industrial needs. However, for various clean rooms and super clean rooms, we need to install

  • High Efficiency Particulate Air Filters (HEPA) (H class) and
  • Ultra Low Particulate Air Filters (ULPA) (U class)

While HEPA filters can control contamination up to 0.3 micron (µm), ULPA can even control up to 0.12 micron (µm).

Various filters described above do take care of unwanted particulate contamination in the air. However, it is a very big challenge to control contamination of any matter smaller than particulate size of 0.12 micron (µm) and physically arrest them.

In depth knowledge of adsorption by desiccants help us to control contamination in such critical applications. Adsorption by various desiccants of matters (gases) having molecular diameter fraction of 0.1 micron (µm) in their micro/meso pores (diameter between 0.2 to 1.0 nm (nano meter)) is the starting point of filtration of gaseous contaminants.

Why do we need molecular filtration at such levels (less than 0.12 microns)?

Many of the unwanted gases which contaminate and can cause serious damage resulting in huge losses are either

  1. Odorous
  2. Or corrosive
  3. Or both

These gaseous contaminants are potentially very harmful to humans as well to equipment, especially in an environment of controlled areas housing sensitive equipment like servers in data centers, etc.

  • Some of the environmentally conditioned areas where odorous gases cause loss of productivity are:
    • Animal Research facilities
    • Autopsy rooms in Mortuaries / Hospitals
    • Call Centers (BPO’s) near landfill area, like “Mind Space” in Malad, Mumbai, or near open Sewage line, like in Noida.
  • Some of environmentally conditioned areas where corrosive gases are a cause of loss of productivity /down time of a process industries are:
    • Petrochemical industries
    • Fertilizer industries
    • Paper and pulp industries
    • Medium size server rooms
    • Mission critical facilities e.g. large size data centers

As shown in Table below, gases typically have molecular diameter in the range of 0.0002 micron to 0.001 micron (µm). Unit used for measurement is Angstrom (Å) (1 micron = 10,000 Angstrom)

S. No.
Unwanted Gases
Molecular Diameter
Nature of Gas
Sources of gases in Urban Areas
Sources of gases in Industrial Areas
in Angstrom (Å)
1 H2S

(Hydrogen Sulfide)


34.08 Yes


  • Combustion of fossil fuels
  • Drains
  • Land filled sites
  • Auto emissions
  • Microbiological activities, etc.
  • Wood Pulping
  • Sewage treatment
  • Sulfuric acid manufacturing
  • Fossil fuels processing
  • Electric power plants burning coal as fuel containing sulfur
  • Oil and gas extraction operations, oil refineries, etc.
2 SO2
(Sulfur Dioxide)


64.07 Yes


  • Combustion of fossil fuels
  • Drains
  • Auto emissions
  • Tobacco smoke, etc.
  • Sulfuric acid manufacturing
3 SO3
(Sulfur Trioxide)


80.07 Yes


4 HF
(Hydrogen Fluoride)


20.01 No


  • Combustion of fossil fuels
  • Drains, etc.
  • Fertilizers manufacturing industry
  • Aluminum manufacturing industry
  • Ceramic manufacturing industry
  • Steel manufacturing industry
  • Electronic device manufacturing industries
5 NO2
(Nitrogen Dioxide)
2.3 46.01 Yes Yes
  • Combustion of fossil fuels
  • Auto emissions
  • Microbes, etc.
  • All process industries
6 NH3


2.6 17.03 Yes Yes
  • Microbes
  • Public toilets /drains
  • Refrigeration Equipment
  • Cleaning products, etc.
  • Fertilizer plants
7 Cl2


3.2 70.9 Yes Yes
  • Refuse decomposition
  • Cleaning products, etc.
  • Chlorine manufacturing industries
  • Aluminum manufacturing industries
  • Paper mills
  • Steel industries, etc.
8 O3
2.6 48 No Yes
  • Atmospheric photo chemical processes mainly involve nitrogen oxides
  • Auto emission
  • Electrostatic filters photo copiers and printers, etc.
9 HCl
3.2 36.47 Yes Yes
  • Auto emission
  • Fossils fuels combustion
  • Polymer combustion
  • Oceanic processes
10 CnHn/VOC
(Hydrocarbons/ Volatile Organic Compounds)
4-4.9 Yes Yes
  • Auto emission
  • Tobacco smoke
  • Microbes
  • Paints, etc.
  • All petrochemical and fertilizer industries
  • Paper mills, etc.

This article is going to give more insight into the adverse effects of corrosive gases and potential filtration options currently available in the market.

How to remove corrosive gases?

As shown in figure 3, the process of filtration through adsorption and neutralization through chemical reaction is commonly known as Chemisorption. The air filtration systems remove corrosive gases through the process of adsorption and neutralization.



Adsorption with Chemical Neutralization/Oxidation

  • The process is specific and depends on the chemical nature of both the media and gas
  • The process is instantaneous and irreversible
  • Convert harmful gases to harmless solids

What are the major elements to remove corrosive gases?

  • Granular Media Filters – It is a combination of desiccants impregnated with chemicals like:
  • Activated alumina impregnated with KMnO4
  • Activated carbon and activated alumina impregnated with KOH
  • Activated carbon alone
  • Activated carbon impregnated with H3PO4
  • Honeycomb Chemical Filters – These are desiccant honeycomb matrix impregnated with choice of oxidizing agents and/or alkaline/acidic solutions like:
  • Desiccant honeycomb matrix based chemical filters impregnated with KMnO4
  • Desiccant honeycomb matrix based chemical filters having both metal silicate and activated carbon impregnated with KOH
  • Desiccant honeycomb matrix based chemical filters impregnated with H3PO4

Evolution of Gas Phase Filtration Technologies

The above figure traces how the various types of filters have evolved from carbon media to honeycomb chemical filters.

How to classify the reactive environments?

International Society of Automation (ISA) defined severity levels on account of unwanted gases in instrumentation and control rooms way back in 1985. Keeping in view implementation of ROHS (Restriction Of use of Hazardous Substances) under the directive from EU (European Union) as per 2002/95/EC replacing lead (being carcinogenic) by silver and electronic circuits getting further miniaturized has led to ISA revising 1985 standard in 2013 which is as per below table below:


Angstroms (Å) per 30 days




G1 Mild <300 Å <200 Å Corrosion is NOT a factor in electronic equipment reliability
G2 Moderate 300 Å – 999 Å Effect of corrosion is measurable and may be a factor in electronic equipment reliability
G3 Harsh 1,000 Å – 1,999 Å High probability that corrosive attacks will occur. Should prompt further evaluation and result in environmental controls
GX Severe > 2000 Å Only specially designed and packaged equipment is expected to survive

The ISA standard also defines in terms of gaseous concentration levels as per below table below:

 Concentration of Gases (in ppb) as per ISA – 71.04-2013


G-1 (Mild) G2 (Moderate) G3 (Harsh)

GX (Severe)

H2S <3 <10 <50 >50
SO2 <10 <100 <300 >300
Cl2 <1 <2 <10 >10
NOX <50 <125 <1250 >1250
HF <1 <2 <10 >10
NH3 <500 <10000 <25000 >25000
O3 <2 <25 <100 >100

How to measure severity levels in corrosive environments?

Typically, there are two types of measurement methods as below:

  1. Corrosion Classification Coupons (CCC):

Corrosion classification coupons have two pure metal strips of silver and copper.

These coupons are placed in the room where environment severity has to be measured for a period of 30 days.

The thickness of the layer of corrosion which forms on metal strips determines the “severity level” as per ISA 71.04-2013 standard.

  1. Real time atmospheric corrosion monitors:

These instruments help to access severity levels on real time basis. The real time measurements in typically 24 hours are extrapolated for 30 days to know the severity levels as per ISA standard.

In addition to severity levels due to airborne gaseous contaminants these instruments do also measure room temperature, RH and optionally, the differential pressures, to give the complete corrosion parameters.

Real time atmospheric corrosion monitors can be further classified in two technologies:

* One is based on Quartz Crystal Microbalance (QCM), which measures the rate of increase of corroded metal sensors mass

* The other determines the rate of electrical resistance increase of corroded metal strips

What are the types of equipment available for removing corrosive gases?

Equipment for removal of unwanted corrosive gases are broadly classified as under:

  • Thin Bed – For recirculation of air to clean within an enclosed space
  • Deep Bed – These are generally designed to clear fresh air inducted into controlled space for pressurization

The above two concept categories can involve both types of chemical filters:

  • Granular type
  • Honeycomb type

Deep bed and thin bed

Internal view of the system

Working principle of Deep bed and thin bed

How to install the equipment?

Gas Phase filtration systems are typically installed in three ways

  1. Re-circulation option (as shown in figure below),
  2. Pressurization option (as shown in figure below) and
  3. Re-circulation + pressurization option (as shown in figure below).

Re-circulation option system used for server/data center application

Pressurization option system used for cleaning fresh air

Re-circulation + pressurization system type

What are the precautions to be taken for ensuring proper filtration?

  1. Room should be reasonably air tight
  2. Pressurize the room & try to maintain minimum positive pressure of 2.5 mm
  3. Continuously monitor air quality in a controlled environment and at equipment outlet
  4. Regular equipment maintenance services
  5. Avoid Acidic/Chlorinated agents for cleaning
  6. RH-Temperature sensors interlocking with BMS to cut human intervention


Server rooms, data centers and mobile/base switching centers are mushrooming in urban areas. Knowledge of filtration at molecular level (more commonly referred as Gas Phase Filtration System) is helping in keeping such facilities with minimum downtime.

With the increase in automation in process industries, need of protecting their control rooms against corrosion from unwanted gases using Gas Phase Filtration has become the need of the hour.

We hope this article helps in better understanding of basics of air filtration at molecular level and need for gas phase filtration. This article, however, only gives the over view of the dynamics involved.


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