The next most important source of indoor air pollution is allergens. House dust mites (HDM) are very small insects not visible to the naked eye and the commonest source of allergy in the house. They are ubiquitous and thrive in a warm and moist atmosphere. They breed very fast and are very difficult to eradicate. Modern houses present ample breeding spaces for them in the form of carpets, curtains, mattresses, pillows, etc. 

Exposure to HDM can be prevented by the frequent washing of linen and by encasing the mattresses and pillows in a non-permeable cover. Pets form an important part of life for some of us. But they can add plenty of allergens to our indoor atmosphere. Cats are notorious for doing this. Fine particles from feline fur can remain stuck to the upholstery and carpets for a long time even after the removal of the animal and lead to the worsening of asthma and skin allergies. Fortunately, owning to religious and social customs, cats are not popular pets in India. Dogs, however, are quite popular and can be as troublesome. Pets should be kept out of the bedroom and groomed frequently. To remove the fur particles, one has to use vacuum cleaners as the ordinary broom and mop are not effective. 

Moulds, fungi and several other microorganisms thrive in damp condition and can lead to allergies as well as infection. Humidifiers the air-conditioning plants provide an ideal environment for certain types of bacteria and have led to major outbreaks of pneumonia. It is important to regularly clean the coolers, air-conditioners and damp areas of the house such as cupboards, lofts, etc., to minimize this risk. 

Toxic gases can also pollute the indoor environment. Biomass fuels (wood, cow dung, dried plants) and coal, if burned inside, can lead to serve contamination by carbon monoxide (CO). The poor quality of stoves and other cooking or heating appliances that cause incomplete combustion of LPG can also lead to the emission of CO or nitrogen dioxide. Formaldehyde (a gas) can be released from adhesives that are used for fixing carpets, upholstery and also in making plywood and particle-board.

From the passage, we can easily state that home dust mites are very ________.

Are very small and cannot be seen by the naked eye

House dust mites (HDM) are very small insects not visible to the naked eye and the commonest source of allergy in the house.