All of this is caused by fires in Indonesia, slash-and-burn farming to replenish the fields or create new fields. Indonesia claims that many of the fires are started on Palm Oil plantations that are owned by Singaporean or Malaysian companies but then refuses to name the companies. The diplomatic tit-for-tat is all getting quite childish.
You can see the fires:
A few minutes outside in this is enough to give you a crappy feeling in your stomach all day. The government publishes numbers of the Pollutant Standards Index (PSI) [wikipedia.org], but their site only shows today’s numbers [nea.gov.sg]. Wikipedia has the relevant historic numbers, it’s ugly this month: