Relevant limit standards and unit conversion methods for the concentrations of major gaseous pollutants in indoor air

The correlation between indoor air gaseous pollutants and human health, especially the impact of indoor air pollutants such as ozone and nitrogen dioxide on the cardiopulmonary function of vulnerable groups, has attracted widespread attention worldwide.

What are gaseous pollutants?

In air quality science, gaseous pollutants are molecular or vapor-based impurities suspended in the air. These are much smaller than aerosol pollutants (like dust), typically measuring just a few angstroms (1 angstrom = 0.1 nanometer), whereas aerosols are defined as being larger than 0.01 microns. As a result, traditional dust filtration doesn’t capture gaseous pollutants instead, specific gas purification technologies are needed.

Difference between the diameter of gaseous pollutants and aerosol pollutants

Pollutants in the air refer to gaseous components other than the normal components of the air or so-called impure components.

When gaseous pollutants exist in the form of molecules or vapor, the diameter of their molecules is generally very small, ranging from a few angstroms to tens of angstroms. The diameter of aerosol particles is generally considered to be greater than 0.01 microns. To remove aerosol pollutants, dust removal is generally used, while to remove gaseous pollutants, gas purification is generally used without considering dust removal.

Main gaseous pollutants in indoor air

The gaseous pollutants specified in the indoor air quality standards include sulfur dioxide, nitrogen dioxide, carbon monoxide, carbon dioxide, ammonia, ozone, formaldehyde, benzene, toluene, xylene, benzo[a] pyrene and various volatile organic compounds (TVOC).

 Method of expressing gas concentration

There are two ways to express the concentration of pollutants in the gas:

1. Method of expressing mass concentration

The mass concentration is a commonly used expression method, usually expressed in milligrams per cubic meter (mg/m³). For trace or trace gas pollutants, it can be expressed in micrograms per cubic meter (μg/m³), 1 mg = 1000 μg.

In order to study the volume changes under different temperature and pressure conditions, the volume is often converted into the volume under standard conditions and expressed in standard cubic meters (Nm ³ ).

The temperature T ₀ of the standard state is 273.1K (0°C) and the pressure P ₀ is 1.013×10 ⁵ Pa (760 mmHg).

Converting the gas volume V1 under actual measurement conditions such as the atmosphere or indoors into the volume V0 under standard conditions is shown in the formula.

P ₁ V ₁ /T ₁ = P ₀ V ₀ /T ₀

Wherein, P ₁ and T ₁ are the gas volume and temperature values under actual measurement.

[Example] The temperature T1 of the sampling air environment is 293.1K (20 ℃ ), the atmospheric pressure P1 is 1.0063 × 10 ⁵ Pa (755mmHg), the sampling flow rate of the sampler is 1.5m ³ , and the amount of floating dust collected is 0.3mg. Calculate the concentration of floating dust in the air when converted to standard conditions.

Solution: Given that T ₁ = 293.1K, P ₁ = 1.0063 × 10 ⁵ Pa, V ₁ = 1.5 m ³ ;

Under standard conditions, P ₀ = 1.013×10 ⁵ Pa, T ₀ = 273.1K. The volume of the sampled air converted into the volume V ₀ under standard conditions is:

V ₀ =P ₁ V ₁ T ₀ /P ₀ T ₁ =1.0063×10 ⁵ ×1.5×273.1/1.013×10 ⁵ ×293.1=1.388Nm³

The concentration of floating dust in the air is converted into the concentration C0 under standard conditions :

C ₀ =0.3mg/1.388m ³ =0.216mg/m ³

(II) Method of expressing volume concentration

The volume concentration expression method is the commonly used method for expressing gas pollutant concentration internationally. It is generally expressed in cm ³ or m ^3. That is, the volume cubic number of pollutants contained in each cubic meter of gas. Because 1m ³ = 10 ⁶ cm ³ , that is, 1 cubic centimeter is equal to one millionth of a meter, so in actual use, it is expressed in parts per million, that is, ppm. This is the expression method of volume concentration. The concentration of trace or trace pollutants in the air is sometimes expressed in ppb or ppt.

1ppb＝10^-3 ppm ＝10^-9 ; 1ppt＝ 10^-3 ppb＝ 10^-6 ppm= 10^-12 .

Conversion method of gas mass concentration and volume concentration

Conversion method between mass concentration and volume concentration

mass concentration unit mg/ m³ into volume concentration unit ppm is:

C _m = MC _ppm /22.4

for converting the volume concentration unit ppm into the mass concentration unit mg/m3 is :

C _ppm =22.4C _ppm /M

Where:

C _m ——mass concentration of pollutant, in mg/m ³ ;

C _ppm ——volume concentration of pollutant, in ppm;

M – molecular weight of the pollutant.

[Example] After sampling and testing, the mass concentration of sulfur dioxide converted to standard conditions is 0.2 mg/m ^3. How many ppm should it be converted into volume concentration?

Solution: The molecular weight of sulfur dioxide is 64.

C _ppm =22.4C _ppm /M＝22.4×0.2/64＝0.07ppm

[Example] The new maximum permissible concentration standard of ozone in the air approved by the US Congress is 0.05ppm. What is the mass concentration in mg/ m³ ?

Solution: It is known that the molecular weight of ozone is 48, then

C _m = MC _ppm /22.4 =48×0.05÷22.4＝0.1mg/m ³

The ozone concentration standard issued by UL in the United States is 0.005 ppm

C _m = MC _ppm /22.4 =48×0.005÷22.4＝0.01mg/m ³

[Example] The molecular weight of formaldehyde is 29. The maximum allowable concentration of formaldehyde in the air of China’s indoor air quality standard is 0.08 mg/m ³ . Convert it to volume concentration

C_ppm = Cm × 22.4 ÷ 29 = 0.06ppm

According to the American WELL standard, the maximum allowable concentration of formaldehyde in the air is 0.027ppm; converted to body mass concentration:

C_m= M Cppm /22.4＝29×0.027÷22.4＝0.035mg/m ³

 The European Union (EU) has increasingly stringent management of indoor air quality. Although there is no mandatory unified “indoor air quality concentration limit” regulation at the EU level, the European Commission and its relevant agencies (such as the European Chemicals Agency ECHA and the European Environment Agency EEA) have jointly issued a series of recommended limit standards, and some member states (such as Germany, France, and Finland) have also formulated national standards with mandatory or guiding significance. These limits are usually based on the World Health Organization (WHO) and internationally recognised toxicological research results.

 Concentration limit standards for major gaseous pollutants in indoor air

The following is a summary of the concentration limits (recommended values or reference values) for major indoor air gaseous pollutants in the European Union and its member states, in μg /m³ (micrograms per cubic meter):

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