SINGAPORE – Rain provided respite from the haze on Monday (Sept 23), and the weatherman says to expect more wet weather over the next few days.

But readers of The Straits Times have asked: “When rain mixes with haze, does this create a poisonous mix?”

“No, it isn’t harmful at all. People shouldn’t worry if a downpour catches them without an umbrella in the following days,” air quality scientist Erik Velasco told The Straits Times.

The amount of toxic pollutants trapped by the raindrops would not pose any health risk to humans – even if high Pollutant Standards Index (PSI) levels were breached.

“The water used to mop our houses ends much more polluted than the rainwater during a haze event like the current one,” said Dr Velasco.

In fact, downpours can help to temporarily clean the air during haze episodes. Drizzles would not do much.

But is there a difference in the composition between regular rainwater and rainwater in a hazy environment?

“We can expect a much larger amount of pollutants in rainwater in places impacted by severe smoke-haze,” said Dr Velasco.

“The term ‘acid rain’ is used for polluted rainwater. The balance between ions (charged particles, both positive and negative) and cations (positive particles) becomes different, and consequently modifies the rainwater acidity.”

Though these changes in the water pH do not pose any risk to people, it affects ecosystems.

Owners of outdoor fish ponds, especially small ponds, should be careful with such rainwater. The difference in cations and ions may modify the water pH and kill the fish.

In the long term, polluted rain also ends up poisoning soil and water bodies like streams and lakes. But, on a short-term basis, such polluted rain may provide additional nutrients to plants.

“It’s like when we consume pizzas and candies – it tastes good and gives us additional calories, but in the long term it brings a long list of health ailments,” said Dr Velasco.

It is not recommended to drink or use such water for cooking and washing vegetables.

The National Environment Agency says PSI readings of 50 and below is of “good” air quality, while 51-100 is “moderate” and 101-200 “unhealthy”.

At 5pm on Tuesday , the PSI for all regions in Singapore was above 80, with the south recording the highest at 96 and the north the lowest at 81.