As temperatures have dropped to the lowest in years in the northern part of the country, people are reluctant to open their windows for a breath of fresh air.

And it is not only the smoggy outdoor air that urbanites are worried about. They also fear that indoors, they'd be exposed to more harmful gases and chemical vapours being released from furniture, building materials, cooking appliances as well as second-hand smoke.

How to clean the indoor air? A few pots of green house plants may be a good and practical answer.

Using plants as natural air purifiers is a way to combat the worsening global problem of indoor air pollution. Nowadays, growing plants indoors to help purify the air is a popular practice among urban Chinese families.

Those who just moved into new apartments or have their living quarters newly refurbished would especially rush to flower and plant markets to select a few pots of indoor plants. They believe the plants, especially the green broadleaves, would create a healthier living environment.

Many are ready to share their experiences, citing that the first batch of green plants they grew when they first moved into their new homes actually withered and "sacrificed their lives" for "standing up against the pollutants."

On weekends at Yuquanying Flowers and Plants Market, the biggest of its kind in Beijing, visitors come in droves. Most stop in front of green indoor plant stalls to inquire about the varieties and prices.

According to Liu Yuwen, a vendor in the market, most people came here to select plants with the purpose of improving the air quality in their rooms.

Zhang Li and her husband are typical customers. The couple arrived here early this month to search for what she called "detoxifying" plants. They just moved into their new apartment, where they felt quite uncomfortable with the smell in the rooms.

However, with so many plants out there, the couple is stumped in having to make a decision. After a round of questions, they finally settled on a pot of heijingang, or the rubber tree plant.

"It looks easier to maintain," Zhang said. "More importantly, the vendor told me that it has a strong air purifying ability."

Natural air purifier

Liu said plants like ivy, aloe, African violet, cactus, huangjinge (also called Luluo or Devil's ivy in English), Pachira, diaolan (spider plant), tortoiseshell bamboo, yellow palm, purpleblow maple, rubber plant sell the best.

The price of indoor plants is generally not high, varying between 10 to 100 yuan (US$1.2 to 12) for a pot.

Indoor air pollution has become an increasing health hazard over the past 20 years worldwide, as building construction has focused on energy efficiency by tightly sealing the buildings. It is considered to be the main culprit for increases in asthma, allergy and other related illnesses.

The chemical vapour may come from the synthetic building materials, interior decoration materials, as well as furniture, according to Song Guangsheng, chief engineer with China Indoor Environment Monitoring Committee.

Conventional systems to filter or ventilate the indoor environment do not adequately nor efficiently remove these toxins from homes or offices, according to Song.

The earliest research on indoor plants' air purifying effects was conducted by scientists from the United States National Aeronautics and Space Administration (NASA), who initially hoped to filter the air inside the space vehicles.

Their research, led by Dr Bill Wolverton, has shown that living, green and flowering plants can not only remove several toxic chemicals from the air indoors but also improve air quality by giving off water vapor and releasing oxygen.

Wolverton, now retired and running Wolverton Environmental Services Inc (www.wolvertonenvironmental.com), has written a few guidebooks about indoor plants. One book, entitled "How to Grow Fresh Air : 50 House Plants that Purify Your Home or Office," has received some 15 online reviews at www.amazon.com.

The findings of Wolverton and his colleagues also contributed to more discoveries from researchers from Germany and France, who identified more species of indoor plants capable of cleaning the air.

"However, there remained a void in research in China for the actual purifying effects of the indoor plants sold in the market," Song said.

To fill the research void, China Indoor Environment Monitoring Committee has been collaborating for the past few years with Yuquanying Flowers and Plants Market to evaluate the abilities of the common indoor plants in absorbing chemicals, such as benzene, formaldehyde and ammonia.

To test the cleaning capacity more accurately, the experts calculate how much chemicals per square metre of leaves area of each indoor plant would absorb within 24 hours. The researchers have proved that most of common house plants in the market can absorb ammonia and formaldehyde, and some of them can also absorb benzene.

Ivy has "performed" better than other plants in absorbing formaldehyde and benzene.

Plant precautions

While more research is done to prove the plants' air-purifying power, Song advised that people consult professional indoor environment evaluation workers to test the air quality in their rooms, especially when they move into newly refurbished homes.

In the newly decorated houses, indoor air pollution can be detected simply by the smell. However, after people live there for some time, their sense of smell will not be as sensitive as before, Song said.

"When they know what major pollutants are in their rooms, they can select specific indoor plants and achieve better effect," Song said.

However, people should refrain from cultivating too many plants in the bedrooms, Song warned. Plants would release too much carbon dioxide at night, which is harmful to people asleep.

The number of plants put in the rooms is directly related to their sizes. Generally, experts suggest two pots of 1.5-metre-tall plants will clean the air in a room of 10 square meters.

Safety is another issue people should pay attention to when they are introducing indoor plants. Some plants, such as evening primrose and tulip, have a strong fragrant smell, which can irritate the respiratory tracts. They are especially harmful to people suffering from asthma.

When people touch plants, such as China rose, geranium and redbud, their skins can also have an allergic reaction, with itching and even red measles.

Experts suggest that families with children should avoid plants like cactus, even though it, too, is deemed a nice plant for cleaning the air.

(China Daily December 15, 2005)